Biology lesson 2

similarities of DNA and RNA

> Both are nucleic acids > Both are composed of nucleotides > Both have a sugar-phosphate backbone > Both have four different types of bases

Analyzing the Chromosomes

A karyotype is a visual display of pairs of chromosomes arranged by size, shape, and banding pattern. Any cell in the body except red blood cells, which lack a nucleus, can be a source of chromosomes for karyotyping. In adults, it is easiest to use white blood cells separated from a blood sample for this purpose. In fetuses, whose chromosomes are often examined to detect a syndrome, cells can be obtained by either amniocentesis or chorionic villus sampling. For example, the karyotype of a person who has Down syndrome usually has three copies of chromosome number 21 instead of two Amniocentesis is a procedure for obtaining a sample of amniotic fluid from the uterus of a pregnant woman. A long needle is passed through the abdominal and uterine walls to withdraw a small amount of fluid, which also contains fetal cells (Fig. 13.8a). Tests are done on the amniotic fluid, and the cells are cultured for karyotyping. Karyotyping the chromosomes may be delayed as long as 4 weeks, so that the cells can be cultured to increase their number. Blood tests and the age of the mother are considered when determining whether the procedure should be done. There is a slight risk of spontaneous abortion (about 0.6%) due to amniocentesis, with the greatest risk occurring in the first 15 weeks of pregnancy. Chorionic villus sampling (CVS) is a procedure for obtaining chorionic villi cells in the region where the placenta will develop. This procedure can be done as early as the fifth week of pregnancy. A long, thin suction tube is inserted through the vagina into the uterus (Fig. 13.8b). Ultrasound, which gives a picture of the uterine contents, is used to place the tube between the uterine lining and the chorionic villi. Then, a sampling of the chorionic villi cells is obtained by suction. The cells do not have to be cultured, and karyotyping can be done immediately. But testing amniotic fluid is not possible, because no amniotic fluid is collected. Also, CVS carries a greater risk of spontaneous abortion than amniocentesis—0.7% compared with 0.6%. The advantage of CVS is getting the results of karyotyping at an earlier date. After a cell sample has been obtained, the cells are stimulated to divide in a culture medium. A chemical is used to stop mitosis during metaphase when chromosomes are the most highly compacted and condensed. The cells are then killed, spread on a microscope slide, and dried. In a traditional karyotype, stains are applied to the slides, and the cells are photographed. Staining causes the chromosome to have dark and light cross-bands of varying widths, and these can be used, in addition to size and shape, to help pair up the chromosomes. Today, technicians use fluorescent dyes and computers to arrange the chromosomes in pairs. Following genetic testing, a genetic counselor can explain to prospective parents the chances that a child of theirs will have a disorder that runs in the family. If a woman is already pregnant, the parents may want to know whether the unborn child has the disorder. If the woman is not pregnant, the parents may opt for testing of the embryo or egg before she does become pregnant, as described shortly. Testing depends on the genetic disorder of interest. In some instances, it is appropriate to test for a particular protein, and in others, to test for the mutated gene.

law of segregation

After doing one-trait crosses, Mendel arrived at his first law of inheritance, called the law of segregation, which is a cornerstone of the particulate theory of inheritance. The law of segregation states the following: - Each individual has two factors for each trait. - The factors segregate (separate) during the formation of the gametes. - Each gamete contains only one factor from each pair of factors. - Fertilization gives each new individual two factors for each trait.

binary fission

binary fission process involves a simple splitting of the cell into two new cells (In bacteria, and some protists such as amoebas). Binary fission, is a form of asexual reproduction because it produces new cells that are identical to the original parent cell.

Gene Mutations

A gene is a sequence of DNA bases that codes for a cellular product, most often a protein (see Section 11.2). The information within a gene may vary slightly—these variations are called alleles. In our discussion of patterns in inheritance (see Section 10.1), we examined a few examples of alleles in traits (for instance, those associated with flower color or wing shape). However, many alleles code for variations in proteins that are not easily observed, such as susceptibility to a medication. How are new alleles formed? A new allele is the result of a mutation, or a change in the nucleotide sequence of DNA. Mutations may have negative consequences, as is the case with the alleles for cystic fibrosis (see Section 10.2) However, mutations can increase the diversity of organisms by creating an entirely new gene product with a positive function for the organism. In fact, mutations play an important role in the process of evolution

Genome Editing

A relatively new advance in DNA technology is genome editing, the targeting of specific sequences in the DNA for removal or replacement. There are several methods by which editing may be done; the most widely used is called CRISPR (clustered regularly interspaced short palindromic repeats). CRISPR was first discovered in prokaryotes, where it acts as a form of immune defense against invading viruses. Viruses function by inserting their DNA into host cells, causing those cells to form new viruses (see Section 17.1). The CRISPR system is based on an endonuclease enzyme called Cas9, which is capable of identifying specific sequences of nucleotides in the genomic DNA of the invading virus and breaking both of the DNA strands, thus inactivating the virus. Cas9 identifies the specific nucleotides to be cut using a guide RNA molecule that complementary base-pairs to the genomic DNA sequence (Fig. 12.4). To protect the bacteria from Cas9 activity against its own DNA, a sequence called PAM (which is not found in bacterial cells) must be adjacent to the target DNA sequence. The CRISPR system can be used by researchers to target a specific sequence of nucleotides, in almost any organism, for editing. If the genomic sequence of the target is known, a complementary RNA strand can be used by Cas9 to produce a break in the DNA. This break can be used to inactivate the gene and thus study the role of the gene in the cell, or Cas9 can act as a form of molecular scissors to insert new nucleotides at specific DNA locations. CRISPR and other genome editing technologies continue to develop. Scientists are investigating ways of making the processes more efficient, as well as new applications for genome editing in humans and other organisms. The events of the cell cycle and DNA replication ensure that every cell of the body receives a copy of all the genes. This means that every one of your cells has the potential to become a complete organism. While some cells, such as stem cells, retain their ability to form any other type of cell, most other cells differentiate, or specialize, to become specific types of cells, such as muscle cells. This specialization is based on the expression of certain groups of genes at specific times in development. One of the best ways to understand how specialization influences the fate of a cell is to take a look at the processes of reproductive and therapeutic cloning.

Genetic Disorders of Interest

Although many conditions are caused by interactions of genes and the environment (see Section 10.3), there are many human disorders caused by single gene mutations. A few examples of autosomal disorders are discussed in this section. Some of these disorders are recessive, and therefore an individual must inherit two affected alleles before having the disorder. Others are dominant, meaning that it takes only one affected allele to cause the disorder.

Inversion

An inversion occurs when a segment of a chromosome is turned 180° (Fig. 13.6). You might think this is not a problem because the same genes are present, but the reversed sequence of alleles can lead to altered gene activity if it disrupts the control of gene expression. Inversions usually do not cause problems, but they can lead to an increased occurrence of abnormal chromosomes during sexual reproduction. Crossing-over between an inverted chromosome and the noninverted homologue can lead to recombinant chromosomes that have both duplicated and deleted segments. This happens because alignment between the two homologues is only possible when the inverted chromosome forms a loop

Testing the Embryo and Egg

As discussed in Section 29.2, in vitro fertilization (IVF) is carried out in laboratory glassware. A physician obtains eggs from the prospective mother and sperm from the prospective father and places them in the same receptacle, where fertilization occurs. Following IVF, now a routine procedure, it is possible to test the embryo. Prior to IVF, it is possible to test the egg for any genetic defect. In any case, only normal embryos are transferred to the uterus for further development.

Ex Vivo Gene Therapy

Children who have severe combined immunodeficiency (SCID) lack the enzyme adenosine deaminase (ADA), which is involved in the maturation of cells that produce antibodies. In order to carry out gene therapy to treat this disorder, bone marrow stem cells are removed from the blood and infected with a virus that carries a normal gene for the enzyme. Then the cells are returned to the patient. Bone marrow stem cells are preferred for this procedure, because they divide to produce more cells with the same genes. Patients who have undergone this procedure show significantly improved immune function associated with a sustained rise in the level of ADA enzyme activity in the blood. Ex vivo gene therapy is also used in the treatment of familial hypercholesterolemia, a genetic disorder in which high levels of plasma cholesterol make the patient subject to a fatal heart attack at a young age. A small portion of the liver is surgically excised and then infected with a retrovirus containing a normal gene for a cholesterol receptor before the tissue is returned to the patient. Several patients have experienced lowered plasma cholesterol levels following this procedure. Some cancers are being treated with ex vivo gene therapy procedures. In one procedure, immune system cells are removed from a cancer patient and genetically engineered to display tumor antigens. After these cells are returned to the patient, they stimulate the immune system to kill tumor cells.

Cystic Fibrosis

Cystic fibrosis is an autosomal recessive disorder that occurs among all ethnic groups, but it is the most common lethal genetic disorder among Caucasians in the United States. Research has demonstrated that chloride ions (Cl-) fail to pass through a plasma membrane channel protein in the cells of these patients (Fig. 10.12a). Ordinarily, after chloride ions have passed through the membrane, sodium ions (Na+) and water follow. It is believed that the lack of water passing out of the cells then causes abnormally thick mucus in the bronchial tubes and pancreatic ducts, thus interfering with the function of the lungs and pancreas. To ease breathing in affected children, the thick mucus in the lungs must be loosened periodically, but still the lungs become infected frequently. Clogged pancreatic ducts prevent digestive enzymes from reaching the small intestine, and to improve digestion, patients take digestive enzymes mixed with applesauce before every meal.

before mitosis begins

DNA has been replicated. Each chromatid contains a double helix of DNA, and each chromosome consists of two sister chromatids attached at a centromere. Notice that some chromosomes are colored red and some are colored blue. The red chromosomes were inherited from one parent, the blue chromosomes from the other parent.

Down Syndrome

Down syndrome, also called trisomy 21, is a condition in which an individual has three copies of chromosome 21 (Fig. 9.8). In most instances, the egg contained two copies of this chromosome instead of one. However, in around 20% of cases, the sperm contributed the extra chromosome 21. Down syndrome is easily recognized by the following characteristics: short stature; an eyelid fold; stubby fingers; a wide gap between the first and second toes; a large, fissured tongue; a round head; a palm crease; and, at times, mental disabilities, which can sometimes be severe. The chance of a woman having a Down syndrome child increases rapidly with age, starting at about age 40. The frequency of Down syndrome is 1 in 800 births for mothers under 40 years of age and 1 in 80 for mothers over 40. However, since women under 40 have more children as a group, most Down syndrome babies are born to them than to women over 40.

Testing Fetal Cells

Fetal cells can be tested for various genetic disorders. If the fetus has an incurable disorder, the parents may wish to consider an abortion. For testing purposes, fetal cells may be acquired through amniocentesis or chorionic villus sampling, as described earlier in this section. In addition, fetal cells may be collected from the mother's blood. As early as 9 weeks into the pregnancy, a small number of fetal cells can be isolated from the mother's blood using a cell sorter. Whereas mature red blood cells lack a nucleus, immature red blood cells do have a nucleus, and they have a shorter life span than mature red blood cells. Therefore, if nucleated fetal red blood cells are collected from the mother's blood, they are known to be from this pregnancy. Only about one of every 70,000 blood cells in a mother's blood are fetal cells, and therefore the polymerase chain reaction (PCR) is used to amplify the DNA from the few cells collected. The procedure poses no risk to the fetus.

Testing for Genes Associated with Cancer

Genetic tests can detect the presence of specific alleles in many of the genes mentioned in this section. The advances in personal genomics are also allowing people to be able to be screened for specific genes associated with a family history of cancer (see Section 13.3) Persons who have inherited certain alleles of these genes may decide to have additional testing done, or sometimes elective surgery, to detect the presence of cancer.

Testing the Embryo

If prospective parents are carriers for one of the genetic disorders discussed earlier, they may want assurance that their offspring will be free of the disorder. Genetic diagnosis of the embryo will provide this assurance. Following IVF, the zygote (fertilized egg) divides. When the embryo has six to eight cells, one of these cells can be removed for diagnosis, with no effect on normal development (Fig. 13.11). Only embryos that test negative for the genetic disorders of interest are placed in the uterus to continue developing. So far, thousands of children worldwide have been born free of alleles for genetic disorders that run in their families following embryo testing. In the future, embryos that test positive for a disorder could be treated by gene therapy, so that those embryos, too, would be allowed to continue to term.

Ribosomal RNA

In eukaryotic cells, ribosomal RNA (rRNA) is produced in the nucleolus of the nucleus, where a portion of DNA serves as a template for its formation. Ribosomal RNA joins with proteins made in the cytoplasm to form the subunits of ribosomes, one large and one small. Each subunit has its own mix of proteins and rRNA. The subunits leave the nucleus and come together in the cytoplasm when protein synthesis is about to begin. Proteins are synthesized at the ribosomes, which look like small granules in low-power electron micrographs. Ribosomes in the cytoplasm may be free floating or in clusters called polyribosomes. Often, they are found attached to the edge of the endoplasmic reticulum (ER). Proteins synthesized by ribosomes attached to the ER normally are used by the ER. Proteins synthesized by free ribosomes or polyribosomes are used in the cytoplasm; a protein is carried in a transport vesicle to the Golgi apparatus for modification and then transported to the plasma membrane, where it can leave the cell.

Environmental Influences

In many cases, environmental effects also influence the range of phenotypes associated with polygenic traits. This interaction produces a multifactorial trait. For example, in the case of height (Fig. 10.18), differences in nutrition are associated with variations in the height phenotype. Another excellent example of a multifactorial trait in humans is skin color. Our skin is responsible for assisting in the production of vitamin D, which acts as a hormone to increase bone density. In response to UV radiation from the sun, cells in the skin called melanocytes produce a protective pigmentation called melanin. In northern geographic areas (such as northern Europe), less melanin is produced so that vitamin D production is increased. In geographic areas closer to the equator, more melanin is produced to protect the skin from UV damage. In addition to environmental influences, skin color is a polygenic trait that is influenced by a collection of over 100 different genes. For our discussion here, we will use a simple model of only three pairs of alleles (Aa and Bb and Cc). In this model, each dominant allele represents an allele that makes a contribution to the pigment to the skin. When a very dark person reproduces with a very light person, the children have medium-brown skin. When two people with the genotype AaBbCc reproduce with one another, individuals may range in skin color from very dark to very light. The distribution of these phenotypes typically follows a bell-shaped curve, meaning that few people have the extreme phenotypes and most people have the phenotype that lies in the middle. A bell-shaped curve is a common identifying characteristic of a polygenic trait The interaction of the environment (sunlight exposure) with these genotypes produces additional variations in the phenotype. For example, individuals who are AaBbCc may vary in their skin color, even though they possess the same genotype, and several possible phenotypes fall between the two extremes. Temperature is another example of an environmental factor that can influence the phenotypes of plants and animals. Primroses have white flowers when grown above 32°C but red flowers when grown at 24°C. The coats of Siamese cats and Himalayan rabbits are darker in color at the ears, nose, paws, and tail. Himalayan rabbits are known to be homozygous for the allele ch, which is involved in the production of melanin. Experimental evidence indicates that the enzyme encoded by this gene is active only at a low temperature, and therefore black fur occurs only at the extremities where body heat is lost to the environment. When the animal is placed in a warmer environment, new fur on these body parts is light in color. Many genetic disorders, such as cleft lip and/or palate, clubfoot, congenital dislocations of the hip, hypertension, diabetes, schizophrenia, and even allergies and cancers, are most likely multifactorial because they are likely due to the combined action of many genes plus environmental influences. The relative importance of genetic and environmental influences on a phenotype can vary, and often it is a challenge to determine how much of the variation in the phenotype may be attributed to each factor. This is especially true for complex polygenic traits where there may be an additive effect of multiple genes on the phenotype. If each gene has several alleles, and each allele responds slightly differently to environmental factors, then the phenotype can vary considerably. Multifactorial traits are a real challenge for drug manufacturers, since they must determine the response to a new drug based on genetic factors (for example, the ethnic background of the patient) and environmental factors (such as diet). In humans, the study of multifactorial traits contributes to our understanding of the nature versus nurture debate. Researchers are trying to determine what percentage of various traits is due to nature (inheritance) and what percentage is due to nurture (the environment). Some studies use twins separated from birth, because if identical twins in different environments share a trait, that trait is most likely inherited. Identical twins are more similar in their intellectual talents, personality traits, and levels of lifelong happiness than are fraternal twins separated from birth. NASA conducted an experiment involving the astronaut twins Scott and Mark Kelly on the International Space Station to study how the space environment influences genetics. Biologists conclude that all behavioral traits are partly determined by inheritance and that the genes for these traits act together in complex combinations to produce a phenotype that is modified by environmental influences.

The Modern Interpretation of Mendel's Work

In the early twentieth century, scientists noted the parallel behavior of Mendel's particulate factors and chromosomes and proposed the chromosomal theory of inheritance, which states that chromosomes are carriers of genetic information. Today, we recognize that each trait is controlled by alleles (alternate forms of a gene) that occur on the chromosomes at a particular location, called a locus. The dominant allele is so named because of its ability to mask the expression of the other allele, called the recessive allele. In many cases, the dominant allele is identified by an uppercase (capital) letter, and the recessive allele by the same letter but lowercase (small). While the alleles on one homologue can be the alternates of the alleles on the other homologue (Fig. 10.5), the sister chromatids have the same types of alleles. These alleles act as the blueprints for traits at the physiological and cellular level. We now know that these instructions may be modified slightly by environmental influences (see Section 10.3) and by regulation of gene expression

Reproductive and Therapeutic Cloning

In reproductive cloning, the desired end is an individual that is exactly like the original individual. The cloning of plants has been routine for some time and has been responsible for much of the success of modern agriculture. The cloning of some animals, such as amphibians, has been underway since the 1950s. However, at one time it was thought that the cloning of adult mammals would be impossible because investigators found it difficult to have the nucleus of an adult cell "start over," even when it was placed in another egg that had had its own nucleus removed (an enucleated egg cell). In March 1997, Scottish investigators announced they had cloned a Dorset sheep, which they named Dolly. How was their procedure different from all the others that had been attempted? They began in the usual way, by placing an adult nucleus in an enucleated egg cell; however, the donor cells had been starved. Starving the donor cells caused them to stop dividing and go into a resting stage (the G0 stage of the cell cycle). This was the change needed, because nuclei at the G0 stage are open to cytoplasmic signals for the initiation of development (Fig. 12.5). Now it is common practice to clone farm animals that have desirable genetic traits, and even to clone rare animals that might otherwise become extinct. Currently in the United States, no federal funds can be used for experiments to clone humans. While there are advances in the science of cloning, many problems still exist. For example, in the case of Dolly, it took 247 tries before the process was successful. Also, there are concerns that cloned animals may not have the same life expectancy as noncloned animals. Some, but not all, cloned animals have demonstrated symptoms of abnormal aging. For example, Dolly was put down by lethal injection in 2003 because she was suffering from lung cancer and crippling arthritis. She had lived only half the normal life span for a Dorset sheep. In therapeutic cloning, the desired end is not an individual organism but various types of mature cells. The purposes of therapeutic cloning are (1) to learn more about how cell specialization occurs and (2) to provide cells and tissues that could be used to treat human illnesses, such as diabetes, spinal cord injuries, and Parkinson disease. Therapeutic cloning can be carried out in several ways. The most common method is to isolate embryonic stem cells and subject them to treatments that cause them to become particular cell types, such as red blood cells, muscle cells, or nerve cells (Fig. 12.5b). Because embryonic stem cells have the potential to become any other type of cell in an organism, they are said to be totipotent. Eventually, it may be possible to produce entire tissues and organs from totipotent stem cells. Ethical concerns exist about this type of therapeutic cloning because, if the embryo had been allowed to continue development, it would have become an individual. Another way to carry out therapeutic cloning is to use adult stem cells, found in many organs of an adult's body. Adult stem cells are said to be multipotent,since they have already started to specialize and are not able to produce every type of cell in the organism. For example, the skin has stem cells that constantly divide and produce new skin cells, while the bone marrow has stem cells that produce new blood cells. Currently, adult stem cells are limited in the number of cell types that they may become. However, scientists have been able to coax adult stem cells from skin into becoming more like embryonic stem cells by adding only four genes. Researchers are investigating ways of controlling gene expression in adult stem cells, so that they can be used in place of the more controversial embryonic stem cells in fighting disease in humans. Today, bacteria, plants, and animals are genetically engineered to produce biotechnology products. Recall that a genetically modified organism (GMO) is one whose genome has been modified in some way, usually by using recombinant DNA technology. Organisms that have had a foreign gene inserted into their genome are called transgenic organisms.

Genomics and Proteomics

In the twentieth century, researchers discovered the structure of DNA, how DNA replicates, and how protein synthesis occurs. Genetics in the twenty-first century is focusing on genomics, the study of all types of genomes, which consist of genes and intergenic DNA. Researchers now know the sequence of all the base pairs along the lengths of the human chromosomes. The enormity of this accomplishment can be appreciated by knowing that, at the very least, our DNA contains 3.2 billion base pairs and approximately 23,000 genes. Many organisms have even larger genomes. While genes that code for proteins make up only 3-5% of the human genome, we now know that the other regions are also important. Historically considered "junk" DNA, the regions between genes play an important role in generating the small RNA molecules that are involved in gene regulation. Many of these intergenic regions also have evolutionary significance and may provide us with useful hints on the evolution of our species.

autosome

Many traits and disorders in humans, and other organisms, are genetic in origin and follow Mendel's laws. These traits are often controlled by a single pair of alleles on the autosomal chromosomes. An autosome is any chromosome other than a sex (X or Y) chromosome.

Messenger RNA

Messenger RNA (mRNA) is produced in the nucleus of a eukaryotic cell, as well as in the nucleoid region of a prokaryotic cell. DNA serves as a template for the formation of mRNA during a process called transcription. Which DNA genes are transcribed into mRNA is highly regulated in each type of cell and accounts for the specific functions of all cell types. Once formed, mRNA carries genetic information from DNA in the nucleus to the ribosomes in the cytoplasm, where protein synthesis occurs through a process called translation.

In Vivo Gene Therapy

Most cystic fibrosis patients lack a gene that codes for a regulator of a transmembrane carrier for the chloride ion. In gene therapy trials, the gene needed to cure cystic fibrosis is sprayed into the nose or delivered to the lower respiratory tract by adenoviruses or in liposomes. Investigators are trying to improve uptake of this gene and are hypothesizing that a combination of all three vectors might be more successful. Genes are also being used to treat medical conditions such as poor coronary circulation. It has been known for some time that vascular endothelial growth factor (VEGF) can cause the growth of new blood vessels. The gene that codes for this growth factor can be injected alone, or within a virus, into the heart to stimulate branching of coronary blood vessels. Patients who have received this treatment report that they have less chest pain and can run longer on a treadmill. Rheumatoid arthritis, a crippling disorder in which the immune system turns against a person's own body and destroys joint tissue, has recently been treated with in vivo gene therapy methods. Clinicians inject adenoviruses that contain an anti-inflammatory gene into the affected joint. The added gene reduces inflammation within the joint space and lessens the patient's pain and suffering. Clinical trials have been promising, and animal studies have even shown that gene therapy may stave off arthritis in at-risk individuals.

The Spindle

Most eukaryotic cells rely on a spindle, a structure of the cytoskeleton, to pull the chromatids apart. A spindle has spindle fibers made of microtubules that are able to assemble and disassemble. First, the microtubules assemble to form the spindle that takes over the center of the cell and separates the chromatids. Later, the microtubules disassemble.

codominance

Notice that human blood type inheritance is also an example of codominance, another type of inheritance that differs from Mendel's findings because more than one allele is fully expressed. When an individual has blood type AB, both A and B antigens appear on the red blood cells. The two different capital letters signify that both alleles are coding for an antigen.

Structure of DNA

Once researchers knew that DNA was the genetic material, they were racing against time and each other to determine the structure of DNA. They believed that whoever discovered it first would get a Nobel Prize. How James Watson and Francis Crick determined the structure of DNA (and eventually received a Nobel Prize) resembles the solving of a mystery, in which each clue was added to the total picture until the breathtaking design of DNA—a double helix—was finally revealed. To achieve this success, Watson and Crick particularly relied on studies done by Erwin Chargaff and Rosalind Franklin.

Polygenic Inheritance

Polygenic traits are those that are governed by multiple genes, each with several sets of alleles. Each of these alleles has a quantitive effect on the phenotype. Generally speaking, each dominant allele produces a gene product that has an additive effect, while recessive alleles produce little or no effect. The result is a continuous variation of phenotypes, resulting in a distribution of these phenotypes that resembles a bell-shaped curve. The more genes involved, the more continuous the variations and distribution of the phenotypes. One example is human height, where genomic studies have indicated that there may be as many as 700 different alleles that influence phenotypes. The combination of these genes produces minor additive effects and a characteristic bell-shaped distribution of phenotypes

Genetic Testing

Potential parents are becoming aware that many illnesses are caused by abnormal chromosomal inheritance or by gene mutations. Therefore, more couples are seeking genetic counseling, which helps determine the risk of inherited disorders in a family. For example, a couple might be prompted to seek counseling after several miscarriages, when several relatives have a particular medical condition, or if they already have a child with a genetic defect. The counselor helps the couple understand the mode of inheritance, the medical consequences of a particular genetic disorder, and the decisions they might wish to make. Various human disorders may result from abnormal chromosome number or structure. When a pregnant woman is concerned that her unborn child might have a chromosomal defect, the counselor may recommend karyotyping the fetus's chromosomes.

RNA Structure and Function

Ribonucleic acid (RNA) is made up of nucleotides containing the sugar ribose, thus accounting for its name. The four nucleotides that make up an RNA molecule have the following bases: adenine (A), uracil (U), cytosine (C), and guanine (G). Notice that, in RNA, uracil replaces the thymine found in DNA RNA, unlike DNA, is single-stranded, but the single RNA strand sometimes doubles back on itself, allowing complementary base pairing to occur.

Epigenetics

Scientists are beginning to recognize that some changes in gene regulation, especially those occurring at the cellular level, may be inherited from one generation to the next. For example, in X-inactivation (see Fig. 11.21), the pattern of inactivation is inherited as the cell divides. In an XX cell, the inactive chromosome in the first cell is the same in all of its daughter cells. This explains the patches of colored fur on the cat in Figure 11.21; the black patches are all inherited from a cell that expresses only the black allele because the orange is inactivated. This is an example of epigenetic inheritance, or the inheritance of changes in gene expression that are not the result of changes in the sequence of nucleotides on the chromosome. Epigenetic inheritance explains other physiological processes as well, such as genomic imprinting, in which the sex of the individual determines which alleles are expressed in a cell by suppressing genes on chromosomes inherited from the opposite sex. DNA Transcription Transcription in eukaryotes follows the same principles as in bacteria, except that many more regulatory proteins per gene are involved. The occurrence of so many regulatory proteins not only allows for greater control but also brings a greater chance of malfunction. In eukaryotes, transcription factors are DNA-binding proteins that help RNA polymerase bind to a promoter. Several transcription factors are needed in each case; if one is missing, transcription cannot take place. All the transcription factors form a complex that also helps pull double-stranded DNA apart and even acts to position RNA polymerase so that transcription can begin. The same transcription factors, in different combinations, are used over again at other promoters, so it is easy to imagine that, if one malfunctions, the result could be disastrous to the cell. The genetic disorder Huntington disease is a devastating psychomotor ailment caused by a defect in a transcription factor. In eukaryotes, transcription activators are DNA-binding proteins that speed transcription dramatically. They bind to a DNA region, called an enhancer, that can be quite a distance from the promoter. A hairpin loop in the DNA can bring the transcription activators attached to enhancers into contact with the transcription factor complex A single transcription activator can have a dramatic effect on gene expression. For example, investigators have found one DNA-binding protein, MyoD, that alone can activate the genes necessary for fibroblasts to become muscle cells in various vertebrates. Another DNA-binding protein, called Ey, can bring about the formation of not just a single cell type but a complete eye in flies mRNA Processing After transcription, the removal of introns and the splicing of exons occur before mature mRNA leaves the nucleus and passes into the cytoplasm. Alternative mRNA processing is a mechanism by which the same primary mRNA can produce different protein products according to which exons are spliced together to form mature mRNAs

Testing for a Protein

Some genetic mutations lead to disorders caused by a lack of enzyme activity. For example, in the case of methemoglobinemia, it is possible to test for the quantity of the enzyme diaphorase in a blood sample and, from that, determine whether the individual is likely homozygous normal, is a carrier, or has methemoglobinemia. If the parents are carriers, each child has a 25% chance of having methemoglobinemia. This knowledge may lead prospective parents to opt for testing of the embryo or egg, as described later in this section.

Genetic Markers

Testing for a genetic marker relies on a difference in the DNA due to the presence of the abnormal allele. As an example, consider that individuals with sickle-cell disease or Huntington disease have an abnormality in a gene's base sequence. This abnormality in sequence is a genetic marker. The presence of specific genetic markers can be detected using DNA sequencing. Another option is to use restriction enzymes to cleave DNA at particular base sequences (see Section 12.1). The fragments that result from the use of a restriction enzyme may be different for people who are normal than for those who are heterozygous or homozygous for a mutation

Levels of Gene Expression Control

The human body consists of many types of cells, which differ in structure and function. Each cell type must contain its own mix of proteins that makes it different from all other cell types. Therefore, only certain genes are active in cells that perform specialized functions, such as nerve, muscle, gland, and blood cells. Some of these active genes are called housekeeping genes because they govern functions that are common to many types of cells, such as glucose metabolism. But the activity of some genes accounts for the specialization of cells. In other words, gene expression is controlled in a cell, and this control accounts for its specialization (Fig. 11.18). Let's begin by examining a simple system of controlling gene expression in prokaryotes.

Cell plate

The cell plate is simply newly formed plasma membrane that expands outward until it reaches the old plasma membrane and fuses with it. The new membrane releases the molecules that form the new plant cell walls. These cell walls are later strengthened by the addition of cellulose fibrils.

Types and Effects of Mutations

The effects of mutations vary greatly. The severity of a mutation usually depends on whether it affects one or more codons on a gene. In general, we know a mutation has occurred when the organism has a malfunctioning protein that leads to a genetic disorder or to the development of cancer. But many mutations go undetected because they have no observable effect or have no detectable effect on a protein's function. These are called silent mutations Point mutations involve a change in a single DNA nucleotide, and the severity of the results depends on the particular base change that occurs. A single base change can result in a change in the amino acid at that location of the gene. For example, in hemoglobin, the oxygen-transporting molecule in blood, a point mutation causes the amino acid glutamic acid to be replaced by a valine (Fig. 13.2). This changes the structure of the hemoglobin protein. The abnormal hemoglobin stacks up inside the red blood cells, causing them to become sickle-shaped, resulting in sickle-cell disease. A frameshift mutation is caused by an extra or missing nucleotide in a DNA sequence. Frameshift mutations are usually much more severe than point mutations, because codons are read from a specific starting point. Therefore, all downstream codons are affected by the addition or deletion of a nucleotide. For instance, if the letter C is deleted from the sentence THE CAT ATE THE RAT, the "reading frame" is shifted. The sentence now reads THE ATA TET HER AT—which doesn't make sense. Likewise, a frameshift mutation in a gene often renders the protein nonfunctional, because its amino acid sequence no longer makes sense (Fig. 13.2). The movement of a transposon can cause a frameshift mutation.

Meiosis II Compared with Mitosis

The events of meiosis II are like those of mitosis, except that in meiosis II the cells have the haploid number of chromosomes.

Absence of Telomere Shortening

The telomeres at the end of the chromosomes play an important role in regulating cell division. Just as the caps at the ends of shoelaces protect them from unraveling, so telomeres promote chromosomal stability, so that replication can occur. Each time a cell divides, some portion of a telomere is lost; when telomeres become too short, the chromosomes cannot be replicated properly and the cell cycle is stopped. Embryonic cells and certain adult cells, such as stem cells and germ cells, have an enzyme, called telomerase, that can rebuild telomeres. The gene that codes for telomerase is turned on in cancer cells. If this happens, the telomeres do not shorten, and cells divide over and over again. Telomerase is believed to become active only after a cell has already started proliferating wildly.

Biotechnology

The term biotechnology refers to the use of natural biological systems to create a product or achieve some other end desired by humans. Today, genetic engineering allows scientists to modify the genomes of a variety of organisms, from bacteria to plants and animals, to either improve the characteristics of the organism or make biotechnology products. Such modification is possible because decades of research on how DNA and RNA function in cells has allowed for the development of new techniques. These techniques allow scientists not only to clone genes, but also to directly edit the genome of an organism. A genetically modified organism (GMO) is one whose genome has been modified in some way, usually by using recombinant DNA technology. A transgenic organism is an example of a GMO that has had a gene from another species inserted into its genome. We will take a closer look at both GMOs and transgenics in Section 12.3, but first we need to explore some of the DNA techniques that are used in biotechnology.

Testing the DNA

There are several methods of analyzing DNA for specific mutations, including testing for a genetic marker, using a DNA microarray, and direct sequencing of an individual's DNA.

Transfer RNA

Transfer RNA (tRNA) is also produced in the cell nucleus of eukaryotes. Appropriate to its name, tRNA transfers amino acids present in the cytoplasm to the ribosomes. There are 20 different amino acids, and each has its own tRNA molecule. At the ribosome, a process called translation joins the amino acids to form a polypeptide chain.

cellular reproduction

We humans, like other multicellular organisms, begin life as a single cell. However, because of cellular reproduction, we become an organism consisting of trillions of cells in less than 10 months. Even after we are born, cellular reproduction doesn't stop—it continues as we grow, and when we are adults, it replaces worn-out or damaged tissues

Mutation Genes

1. Proto-oncogenes code for proteins that promote the cell cycle and inhibit apoptosis. They are often likened to the gas pedal of a car because they accelerate the cell cycle. 2. Tumor suppressor genes code for proteins that inhibit the cell cycle and promote apoptosis. They are often likened to the brakes of a car Page 137because they slow down the cell cycle and stop cells from dividing inappropriately. They are called tumor suppressors because tumors may occur when mutations cause these genes to become nonfunctional.

Centosome

A centrosome is the primary microtubule organizing center of a cell. In an animal cell, each centrosome has two barrel-like structures, called centrioles, and an array of microtubules called an aster. Plant cells have centrosomes, but they are not clearly visible because they lack centrioles. Centrosome duplication occurs at the start of the S phase of the cell cycle and has been completed by G2. During the first part of the M phase, the centrosomes separate and move to opposite sides of the nucleus, where they form the poles of the spindle. As the nuclear envelope breaks down, spindle fibers take over the center of the cell. Some overlap at the spindle equator, which is midway between the poles. Others attach to duplicated chromosomes in a way that ensures the separation of the sister chromatids and their proper distribution to the daughter cells. Whereas the chromosomes will be inside the newly formed daughter nuclei, a centrosome will be just outside it.

angiogenesis

An actively growing tumor can grow only so large before it becomes unable to obtain sufficient nutrients to support further growth. Additional mutations in cancer cells allow them to secrete factors that promote angiogenesis, the formation of new blood vessels. Additional nutrients and oxygen reach the tumor, allowing it to grow larger. Some modes of cancer treatment are aimed at preventing angiogenesis. The patient's prognosis (probable outcome) depends on (1) whether the tumor has invaded surrounding tissues and (2) whether there are metastatic tumors in distant parts of the body.

Cancer locations

Cancers are classified according to their location. Carcinomas are cancers of the epithelial tissue that lines organs; sarcomas are cancers arising in muscle or connective tissue (especially bone or cartilage); and leukemias are cancers of the blood. In this section, we'll consider some general characteristics of cancer cells and the treatment and prevention of cancer.

differences between DNA and RNA

DNA: - found in nucleus - genetic material - sugar is deoxyribose - bases are A, T, C, G - double-stranded - DNA is transcribed (to give variety of RNA molecules) RNA: - found in nucleus and cytoplasm - helper to DNA - sugar is ribrose - bases are A, U, C, G - single-stranded - mRNA is translated (to make proteins) In general, RNA is a helper to DNA, allowing protein synthesis to occur according to the genetic information that DNA provides. There are three major types of RNA, each with a specific function in protein synthesis. (messenger, transfer, ribosomal)

The Second Division—Meiosis II

Essentially, the events of meiosis II are the same as those of mitosis, except that the cells are haploid. At the beginning of prophase II, a spindle appears while the nuclear envelope fragments and the nucleolus disappears. Dyads are present, and each attaches to the spindle. During metaphase II, the dyads are lined up at the spindle equator, with sister chromatids facing opposite spindle poles. During anaphase II, the sister chromatids of each dyad separate and move toward the poles. Both poles receive the same number and kinds of chromosomes. In telophase II, the spindle disappears as nuclear envelopes form. During cytokinesis, the plasma membrane pinches off to form two complete cells, each of which has the haploid number (n) of chromosomes. The gametes are genetically dissimilar because they can contain different combinations of chromosomes and because crossing-over changes which alleles are together on a chromosome. Because both cells from meiosis I undergo meiosis II, four daughter cells are produced from the original diploid parent cell. Each daughter cell contains a unique combination of genes.

4 phases of mitosis

Prophase: Mitotic phase during which chromatin condenses, so that chromosomes appear. Chromosomes are scattered. Metaphase: Mitotic phase during which chromosomes are aligned at the spindle equator. Anaphase: Mitotic phase during which daughter chromosomes move toward the poles of the spindle. Telophase: Mitotic phase during which daughter cells are located at each pole.

DNA replication

An important event that occurs in preparation for cell division is DNA replication, the process by which a cell copies its DNA. Once this is complete, a full copy of all the DNA may be passed on to both daughter cells by the process of cell division. To make cell division easier, DNA and associated proteins are packaged into a set of chromosomes, which allow the DNA to be distributed to the daughter cells. The packaging of the DNA into chromosomes and the process of DNA replication are the work of proteins and enzymes in the nucleus of the cell.

Alkaptonuria

Black urine disease, or alkaptonuria, is a rare genetic disorder that follows an autosomal recessive inheritance pattern. People with alkaptonuria lack a functional copy of the homogentisate oxygenase (HGD) gene found on chromosome 3. The HGD enzyme normally breaks down a compound called homogentisic acid. When the enzyme is missing, homogentisic acid accumulates in the blood and is passed into the urine. The compound turns black on exposure to air, giving the urine a characteristic color and odor (Fig. 10.13). Homogentisic acid also accumulates in joint spaces and connective tissues, leading to darkening of the tissues and eventually arthritis by adulthood.

The Importance of Meiosis

Meiosis is important for the following reasons: 1. It helps keep the chromosome number constant by producing haploid daughter cells that become the gametes. When a haploid sperm fertilizes a haploid egg, the new individual has the diploid number of chromosomes. 2. It introduces genetic variations because (1) crossing-over can result in different types of alleles on the sister chromatids of a homologue, and (2) every possible combination of chromosomes can occur in the daughter cells.

Meiosis I Compared with Mitosis

The following events distinguish meiosis I from mitosis: - During prophase I of meiosis, synapsis occurs. During synapsis, tetrads form and crossing-over occurs. These events do not occur during mitosis. - During metaphase I of meiosis, tetrads align at the spindle equator, with homologous chromosomes facing opposite spindle poles. The paired chromosomes have a total of four chromatids. During metaphase in mitosis, dyads align separately at the spindle equator. - During anaphase I of meiosis, the homologous chromosomes of each tetrad separate, and dyads (with centromeres intact) move to opposite poles. Sister chromatids do not separate during anaphase I. During anaphase of mitosis, sister chromatids separate, becoming daughter chromosomes that move to opposite poles.

2 processes of cellular reproduction

Cellular reproduction involves two important processes: growth and cell division. - During growth, a cell duplicates its contents, including the organelles and its DNA. - Then, during cell division, the DNA and other cellular contents of the parent cell are distributed to the daughter cells. These terms have nothing to do with gender; they are simply a way to designate the beginning cell and the resulting cells. Both processes are heavily regulated to prevent runaway cellular reproduction, which, as we will see, can have serious consequences.

Homologous Chromosomes

Geneticists and genetic counselors can visualize chromosomes by looking at a picture of the chromosomes called a karyotype (Fig. 9.1). Notice how the chromosomes occur in pairs. This is because we inherit one of each chromosome from each parent. Therefore, both males and females normally have 23 pairs of chromosomes. These twenty-three pairs of chromosomes, or 46 altogether, constitute the diploid (2n) number of chromosomes in humans. Half this number is the haploid (n) number of chromosomes. Notice that the haploid number (n) identifies the number of different chromosomes in a cell, while the diploid number is due to the fact that we inherit one set of 23 from each parent. Of these 23 pairs, 22 of these, called autosomes, are the same in both males and females. The remaining pair are called the sex chromosomes, because they contain the genes that determine gender. The larger sex chromosome is the X chromosome, and the smaller is the Y chromosome. Females have two X chromosomes; males have a single X and Y. The members of a chromosome pair are called homologous chromosomes, or homologues, because they have the same size, Page 147shape, and location of the centromere (Fig. 9.1). When chromosomes are stained and viewed under a microscope, homologous chromosomes have the same characteristic banding pattern. One homologue of each pair was contributed by each parent. Homologous chromosomes contain the same types of genes arranged in the same order. A gene is a set of instructions responsible for a specific trait, for example, whether you have freckles or not. Just as your mother may have freckles and your father may not, each homologue may have different versions of a gene. Alternate versions of a gene for a particular trait are called alleles. The location of the alleles of a gene on homolgous chromosomes is the same, but the information within the alleles may be slightly different. Your mother's allele for freckles is on one homologue, and your father's allele for no freckles is on the other homologue.

Multiple-Allele Traits

In ABO blood group inheritance, three alleles determine the presence or absence of antigens on red blood cells and therefore blood type: IA= A antigen on red blood cells IB= B antigen on red blood cells I= Neither A nor B antigen on red blood cells Each person has only two of the three possible alleles, and both IA and IB are dominant over i. Therefore, there are two possible genotypes for type A blood (IAIAand IAi) and two possible genotypes for type B blood (IBIB and IBi). Type O blood can only result from one genotype (ii) because the i allele is recessive. But IAand IB are fully expressed in the presence of each other. Therefore, if a person inherits one of each of these alleles, that person will have a fourth blood type, AB. Figure 10.17 shows that matings between individuals with certain genotypes can produce individuals with phenotypes different than those of the parents.

Meiosis functions

Meiosis serves two major functions: (1) reducing the chromosome number and (2) shuffling the chromosomes and genes to produce genetically different gametes, called sperm (males) and eggs (females). Since we can't predict which sperm will fertilize a given egg, random fertilization introduces another level of additional variation. In the end, each offspring is unique and has a different combination of chromosomes and genes than either parent. But how does meiosis bring about the distribution of chromosomes to offspring in a way that ensures not only the correct number of chromosomes but also a unique combination of chromosomes and genes? For the answer, let's start by examining the chromosomes.

Pedigrees for Autosomal Disorders

A family pedigree for an autosomal recessive disorder is shown in Figure 10.9. In this pattern, a child may have the recessive phenotype of the disorder even though neither parent is affected. These heterozygous parents are sometimes referred to as carriers because, although they are unaffected, they are capable Page 170of having a child with the genetic disorder. If the family pedigree suggests that the parents are carriers for an autosomal recessive disorder, the counselor might suggest confirming this using the appropriate genetic test. Then, if the parents so desire, it would be possible to do prenatal testing of the fetus for the genetic disorder. Figure 10.9 lists other ways that a counselor may recognize an autosomal recessive pattern of inheritance. Notice that, in this pedigree, cousins III-1 and III-2 are the parents of three children, two of whom have the disorder. Generally, reproduction between closely related individuals is more likely to bring out recessive traits since there is a greater chance that the related individuals carry the same alleles. This pedigree also shows that "chance has no memory"; therefore, each child born to heterozygous parents has a 25% chance of having the disorder. In other words, if a heterozygous couple has four children, it is possible that each child might have the condition. Figure 10.10 shows an autosomal dominant pattern of inheritance. In this pattern, a child can be unaffected even when the parents are heterozygous and therefore affected. Figure 10.10 lists other ways to recognize an autosomal dominant pattern of inheritance. This pedigree illustrates that, when both parents are unaffected, all their children are unaffected. Why? Because neither parent has a dominant gene that causes the condition to be passed on.

Gene Interactions

As we have already noted, genes rarely act alone to produce a phenotype. In many cases, multiple genes may be part of a metabolic pathway consisting of the interaction of multiple enzymes and proteins. An example of a trait determined by gene interaction is human eye color (Fig. 10.20). Multiple pigments are involved in producing eye color, including melanin. While a number of factors (including structure of the eye) control minor variations in eye color (such as green or hazel eyes), a gene called OCA2 is involved in producing the melanin that establishes the brown/blue basis of eye color. Individuals who lack OCA2 completely (as occurs in albinism) have red eyes. A recessive form of OCA2 results in blue eyes in homozygous recessive individuals, whereas a single dominant OCA2 allele produces brown eyes (the most common eye color). However, there is another gene, HERC2, that can override the instructions of the OCA2 gene. If individuals are homozygous for a recessive allele of HERC2,they will have blue eyes regardless of the genotype associated with OCA2. Geneticists call this type of interaction, where one gene can override another, an epistatic interaction.

Cancer

Cancer is a genetic disease caused by a lack of control in the cell cycle. The development of cancer requires several mutations, each propelling cells toward the development of a tumor. These mutations disrupt the many redundant regulatory pathways that prevent normal cells from taking on the characteristics of cancer cells. Because of these cumulative events, it often takes several years for cancer to develop, but the likelihood of cancer increases as we age.

X-Linked Recessive Disorders

Color Blindness Red-green color blindness is a common X-linked recessive disorder (see Fig. 10.24). About 8% of Caucasian men see brighter greens as tans, olive greens as browns, and reds as reddish-browns. A few cannot tell reds from greens at all; they see only yellows, blues, blacks, whites, and grays. Duchenne Muscular Dystrophy Duchenne muscular dystrophy is an X-linked recessive disorder characterized by wasting away of the muscles. The absence of a protein, now called dystrophin, is the cause of the disorder. Much investigative work determined that dystrophin is involved in the release of calcium from the sarcoplasmic reticulum in muscle fibers. The lack of dystrophin causes calcium to leak into the cell, which promotes the action of an enzyme that dissolves muscle fibers. When the body attempts to repair the tissue, fibrous tissue forms (Fig. 10.26), and this cuts off the blood supply, so that more and more cells die. Symptoms such as waddling gait, toe walking, frequent falls, and difficulty in rising may appear as soon as the child starts to walk. Muscle weakness intensifies until the individual is confined to a wheelchair. Death usually occurs by age 20; therefore, affected males are rarely fathers. The recessive allele remains in the population through passage from carrier mother to carrier daughter. As therapy, immature muscle cells can be injected into muscles, and for every 100,000 cells injected, dystrophin production occurs in 30-40% of muscle fibers.

Transcription

During transcription of DNA, a strand of RNA forms that is complementary to a portion of DNA. While all three classes of RNA are formed by transcription, we will focus on transcription to create mRNA. mRNA Is Formed Transcription begins when the enzyme RNA polymerase binds tightly to a promoter, a region of DNA with a special nucleotide sequence that marks the beginning of a gene. RNA polymerase opens up the DNA helix just in front of it, so that complementary base pairing can occur. Then the enzyme adds new RNA nucleotides that are complementary to those in the template DNA strand, and an mRNA molecule results The resulting mRNA transcript is a complementary copy of the sequence of bases in the template DNA strand. Once transcription is completed, the mRNA is ready to be processed before it leaves the nucleus for the cytoplasm. mRNA Is Processed The newly synthesized primary mRNA must be processed in order for it to be used properly. Processing occurs in the nucleus of eukaryotic cells. Three steps are required: capping, the addition of a poly-A tail, and splicing (Fig. 11.12). After processing, the mRNA is called a mature mRNA molecule. The first nucleotide of the primary mRNA is modified by the addition of a cap that is composed of an altered guanine nucleotide. On the 3′ end, enzymes add a poly-A tail, a series of adenosine nucleotides. These modifications provide stability to the mRNA; only mRNA molecules that have a cap and tail remain active in the cell. Most genes in humans are interrupted by segments of DNA that do not code for protein. These portions are called introns because they are intervening segments. The other portions of the gene, called exons, contain the protein-coding regions of the gene. In mRNA splicing, the introns are removed and the exons joined together. The result is a mature mRNA molecule consisting of continuous exons. Scientists now know that the genetic sequences in the introns are not "junk" and that many play a regulatory function in gene expression. Ordinarily, processing brings together all the exons of a gene. In some instances, however, cells use only certain exons rather than all of them to form the mature RNA transcript. The result is a different protein product in each cell. In other words, this alternative mRNA splicing increases the number of protein products that can be made from a single gene. After the mRNA strand is processed, it passes from the cell nucleus into the cytoplasm for translation.

Sex-Linked Alleles

For X-linked traits, the allele on the X chromosome is shown as a letter attached to the X chromosome. For example, following is the key for red-green color blindness, a well-known X-linked recessive disorder: XB= normal vision Xb= color blindness The possible genotypes and phenotypes in both males and females are as follows: XBXB= female who has normal color vision XBXb= carrier female with normal color vision XbXb= female who is color-blind XBY= male who has normal vision XbY= male who is color-blind The second genotype is a carrier female because, although a female with this genotype has normal color vision, she is capable of passing on an allele for color blindness. Color-blind females are rare because they must receive the allele from both parents. Color-blind males are more common because they need only one recessive allele to be color-blind. The allele for color blindness has to be inherited from their mother because it is on the X chromosome. Males inherit their Y chromosome from their father. Now let us consider a mating between a man with normal vision and a heterozygous woman (Fig. 10.24). What is the chance that this couple will have a color-blind daughter? A color-blind son? All the daughters will have normal color vision because they all receive an XB from their father. The sons, however, have a 50% chance of being color-blind, depending on whether they receive an XB or an Xb from their mother. The inheritance of a Y chromosome from their father cannot offset the inheritance of an Xb from their mother. Because the Y chromosome doesn't have an allele for the trait, it can't possibly prevent color blindness in a son. Note in Figure 10.24 that the phenotypic results for sex-linked traits are given separately for males and females.

Review of Gene Expression

Genes are segments of DNA that code for proteins. A gene is expressed when its protein product has been made. Figure 11.17 reviews transcription and mRNA processing in the nucleus, as well as translation during protein synthesis in the cytoplasm of a eukaryotic cell. Note that it is possible for the cell to produce many copies of the same protein at the same time. As soon as the initial portion of mRNA has been translated by one ribosome and the ribosome has begun to move down the mRNA, another ribosome attaches to the same mRNA, forming a complex called a polyribosome (String of ribosomes simultaneously translating regions of the same mRNA strand during protein synthesis.) Some ribosomes remain free in the cytoplasm, and others become attached to rough ER. In the latter case, the polypeptide enters the lumen of the ER by way of a channel, where it can be further processed by the addition of sugars. Transport vesicles carry the protein to other locations in the cell, including the Golgi apparatus, which may modify it further and package it in a vesicle for transport out of the cell or cause it to become embedded in the plasma membrane. Proteins have innumerable functions in cells, from enzymatic to structural. Proteins also have functions outside the cell. Together, they account for the structure and function of cells, tissues, organs, and the organism.

Huntington Disease

Huntington disease is a dominant neurological disorder that leads to progressive degeneration of neurons in the brain (Fig. 10.15). The disease is caused by a single mutated copy of the gene for a protein called huntingtin. Most patients appear normal until they are middle-aged and have already had children, who may also have the inherited disorder. There is no effective treatment, and death usually occurs 10 to 15 years after the onset of symptoms. Several years ago, researchers found that the gene for Huntington disease is located on chromosome 4. A test was developed for the presence of the gene, but few people want to know if they have inherited the gene because there is no cure. But now we know that the disease stems from an unusual mutation. Extra codons cause the huntingtin protein to have a series of extra glutamines. Whereas the normal version of huntingtin has stretches of between 10 and 25 glutamines, mutant huntingtin may contain 36 or more. Because of the extra glutamines, the huntingtin protein changes shape and forms large clumps inside neurons. Even worse, it attracts and causes other proteins to clump with it. One of these proteins, called CBP, helps nerve cells survive. Researchers hope to combat the disease by boosting CBP levels.

Gene Expression

In the early twentieth century, the English physician Sir Archibald Garrod observed that family members often have the same metabolic disorder, and he said it was likely they all lacked the same functioning enzyme in a metabolic pathway. He introduced the phrase "inborn error of metabolism" to describe this relationship. Garrod's findings were generally overlooked until 1940, when George Beadle and Edward Tatum devised a way to confirm his hypothesis. These investigators performed a series of experiments utilizing red bread mold and found that each of their mutant molds was indeed unable to produce a particular enzyme. This led them to propose that one gene directs the synthesis of one enzyme (or protein). Today we know that genes are also responsible for specifying any type of protein in a cell, not just enzymes, so Beadle and Tatum's finding has been modified to "one gene-one polypeptide."

Abnormal Sex Chromosome Number

Nondisjunction during oogenesis or spermatogenesis can result in gametes that have too few or too many X or Y chromosomes. Figure 9.7 can be used to illustrate nondisjunction of the sex chromosomes during oogenesis if we assume that the chromosomes shown represent X chromosomes. Just as an extra copy of chromosome 21 causes Down syndrome, additional or missing X or Y chromosomes cause certain syndromes. Newborns with an abnormal sex chromosome number are more likely to survive than are those with an abnormal autosome number. This is because normally females, like males, have only one functioning X chromosome. The other X chromosome is inactivated by a process called X-inactivation, producing an inactive chromosome called a Barr body. If a person has more than two X chromosomes (for example, an XXX female), multiple X chromosomes may be inactivated in her cells. X inactivation is a form of epigenetic inheritance (see Section 11.2). In humans, the presence of a Y chromosome, not the number of X chromosomes, almost always determines maleness. The SRY (sex-determining region Y) gene located on the short arm of the Y chromosome produces a hormone called testis-determining factor, which plays a critical role in the development of male genitals. No matter how many X chromosomes are involved, an individual with a Y chromosome is a male, assuming that a functional SRY is on the Y chromosome. Individuals lacking a functional SRY on their Y chromosome have Swyer syndrome, and are also known as "XY females." A person with Turner syndrome (45, XO) is a female. The number 45 indicates the total number of chromosomes the individual has, and the O signifies the absence of a second sex chromosome. Turner syndrome females are short, with a broad chest and webbed neck. The ovaries, uterine tubes, and uterus are very small and underdeveloped. Turner females do not undergo puberty or menstruate, and their breasts do not develop (Fig. 9.9a). However, some have given birth following in vitro fertilization using donor eggs. These women usually have normal intelligence and can lead fairly normal lives if they receive hormone supplements. A person with Klinefelter syndrome (47, XXY) is a male. A Klinefelter male has two or more X chromosomes in addition to a Y chromosome. The extra X chromosomes become Barr bodies. In males with Klinefelter syndrome, the testes and prostate gland are underdeveloped. There is no facial hair, but some breast development may occur (Fig. 9.9b). Affected individuals generally have large hands and feet and very long arms and legs. They are usually slow to learn but are not mentally handicapped unless they inherit more than two X chromosomes. As with Turner syndrome, it is best for parents to know as soon as possible that their child has Klinefelter syndrome because much can be done to help the child lead a normal life.

Franklin's X-Ray Diffraction Data

Rosalind Franklin was a researcher at King's College in London in the early 1950s (Fig. 11.3a). She was studying the structure of DNA using X-ray crystallography. When a crystal (a solid substance whose atoms are arranged in a definite manner) is X-rayed, the X-ray beam is diffracted (deflected), and the pattern that results shows how the atoms are arranged in the crystal. First, Franklin made a concentrated, viscous solution of DNA and then saw that it could be separated into fibers. Under the right conditions, the fibers were enough like a crystal that, when they were X-rayed, a diffraction pattern resulted. The X-ray diffraction pattern of DNA shows that DNA is a double helix. The helical shape is indicated by the crossed (X) pattern in the center of the photograph in Figure 11.3b. The dark areas at the top and bottom of the photograph indicate that some portion of the helix is repeated many times.

the cell cycle

The cell cycle is an orderly sequence of stages that take place between the time a new cell has arisen from the division of the parent cell to the point when it has given rise to two daughter cells. It consists of interphase (most of the cell cycle is spent in interphase), the time when the cell performs its usual functions; a period of nuclear division called mitosis; and division of the cytoplasm, or cytokinesis. Embryonic cells complete the entire cell cycle in just a few hours. A rapidly dividing mammalian cell, such as an adult stem cell, typically takes about 24 hours to complete the cell cycle and spends 22 hours in interphase. In order for a cell to reproduce successfully, the cell cycle must be controlled. The importance of cell cycle control can be appreciated by comparing the cell cycle to the events that occur in a washing machine. The washer's control system starts to wash only when the tub is full of water, doesn't spin until the water has been emptied, and so forth. Similarly, the cell cycle's control system ensures that the G1, S, G2, and M phases occur in order and start only when the previous phase has been successfully completed.

Internal and External Signals

The checkpoints of the cell cycle are controlled by internal and external signals. These signals are typically molecules that stimulate or inhibit cellular functions. Researchers have identified a series of internal signals called cyclins. The levels of these proteins increase and decrease as the cell cycle progresses; therefore they act as cellular timekeepers. The appropriate cyclin has to be present at the correct levels for the cell to proceed from the G1 phase to the S phase, and from the G2phase into the M phase. In addition, enzymes called kinases remove phosphate from ATP and add it to another molecule. The addition of the energized phosphate from ATP often acts as an off/on Page 135switch for cellular activities. Kinases are active in the removal of the nuclear membrane and the condensation of the chromosomes early in prophase. Some external signals, such as growth factors and hormones, stimulate cells to go through the cell cycle. Growth factors also stimulate tissue repair. Even cells that are arrested in G0 will finish the cell cycle if growth factors stimulate them to do so. For example, epidermal growth factor (EGF) stimulates skin in the vicinity of an injury to finish the cell cycle, thereby repairing the damage. Hormones act on tissues at a distance, and some signal cells to divide. For example, at a certain time in the menstrual cycle of females, the hormone estrogen stimulates cells lining the uterus to divide and prepares the lining for implantation of a fertilized egg. The cell cycle can be inhibited by cells coming into close contact with other cells. In the laboratory, eukaryotic cells will divide until they line a container in a one-cell-thick sheet. Then they stop dividing, due to a phenomenon termed contact inhibition. Contact inhibition prevents cells from overgrowing within the body. When tissues are damaged, cells divide to repair the damage. But once the tissue has been repaired, contact inhibition prevents cell overgrowth by halting the cell cycle. Some years ago, it was noted that mammalian cells in cell cultures divide about 70 times, and then they die. Cells seem to "remember" the number of times they have already divided, and they stop when they have reached the usual number of cell divisions. It's as if senescence, the aging of cells, is dependent on an internal battery-operated clock that runs down and then stops. We now know that senescence is due to the shortening of telomeres. A telomere is a repeating DNA base sequence (TTAGGG) at the ends of chromosomes that can be as long as 15,000 base pairs. Telomeres have been likened to the protective caps on the ends of shoelaces because they ensure chromosomal stability. Each time a cell divides, a portion of a telomere is lost. When telomeres become too short, the cell is "old" and dies by the process of apoptosis.

Changes in Chromosome Number

The normal number of chromosomes in human cells is 46 (2n = 46), but occasionally humans are born with an abnormal number of chromosomes because the chromosomes fail to separate correctly, called nondisjunction, during meiosis. If nondisjunction occurs during meiosis I, both members of a homologous pair go into the same daughter cell. If it occurs during meiosis II, the sister chromatids will fail to separate and both daughter chromosomes will go into the same gamete (Fig. 9.7). If an egg that ends up with 24 chromosomes instead of 23 is fertilized with a normal sperm, the result is a trisomy, so called because one type of chromosome is present in three copies. If an egg that has 22 chromosomes instead of 23 is fertilized by a normal sperm, the result is a monosomy, so called because one type of chromosome is present in a single copy.

The Human Life Cycle

The term life cycle in sexually reproducing organisms refers to all the reproductive events that occur from one generation to the next. The human life cycle involves two types of nuclear division: mitosis and meiosis During development and after birth, mitosis is involved in the continued growth of the child and the repair of tissues at any time. As a result of mitosis, each somatic (body) cell has the diploid number of chromosomes. During sexual reproduction, meiosis reduces the chromosome number from the diploid to the haploid number in such a way that the gametes (sperm and egg) have one chromosome derived from each homologous pair of chromosomes. In males, meiosis is a part of spermatogenesis, which occurs in the testes and produces sperm. In females, meiosis is a part of oogenesis, which occurs in the ovaries and produces eggs. After the sperm and egg join during fertilization, the resulting cell, called the zygote, again has a diploid number of homologous chromosomes. The zygote then undergoes mitosis and differentiation of cells to become a fetus, and eventually a new human. Meiosis is important because, if it did not halve the chromosome number, the gametes would contain the same number of chromosomes as the body cells and the number of chromosomes would double with each new generation. Within a few generations, the cells of sexually reproducing organisms would be nothing but chromosomes! But meiosis, followed by fertilization, keeps the chromosome number constant in each generation.

The First Division—Meiosis I

To help you recall the events of meiosis, keep in mind what meiosis accomplishes in humans—namely, the production of gametes that have a reduced chromosome number and are genetically different from each other and from the parent cell. During prophase I, the nuclear envelope fragments and the nucleolus disappears as the spindle appears. As the chromosomes condense, homologues undergo synapsis to produce tetrads. Crossing-over between nonsister chromatids occurs during synapsis, and this "shuffles" the alleles on chromosomes. During metaphase I, the tetrads attach to the spindle and align at the spindle equator, with each homologue facing an opposite spindle pole. It does Page 152not matter which homologous chromosome faces which pole; therefore, all possible combinations of chromosomes will occur in the gametes. In effect, metaphase of meiosis I shuffles the chromosomes into new combinations. The homologous chromosomes then separate during anaphase I. Following re-formation of the nuclear envelopes during telophase and cytokinesis, the daughter nuclei are haploid: Each daughter cell contains only one chromosome from each homologous pair. The chromosomes are now dyads, and each still has two sister chromatids. No replication of DNA occurs between meiosis I and II, a period called interkinesis.

Deletions and Duplications

A deletion occurs when a single break causes a chromosome to lose an end piece or when two simultaneous breaks lead to the loss of an internal segment of a chromosome. An individual who inherits a normal chromosome from one parent and a chromosome with a deletion from the other parent no longer has a pair of alleles for each trait, and a syndrome can result. Williams syndrome occurs when chromosome 7 loses a tiny end piece (Fig. 13.3). Children with this syndrome have a turned-up nose, a wide mouth, a small chin, and large ears. Although their academic skills are poor, they exhibit excellent verbal and musical abilities. The gene that governs the production of the protein elastin is missing, and this affects the health of the cardiovascular system and causes their skin to age prematurely. Such individuals are very friendly but need an ordered life, perhaps because of the loss of a gene for a protein that is normally active in the brain. Cri du chat (cat's cry) syndrome occurs when chromosome 5 is missing an end piece. The affected individual has a small head, mental disabilities, and facial abnormalities. Abnormal development of the glottis and larynx results in the most characteristic symptom—the infant's cry resembles that of a cat. In a duplication, a chromosome segment is repeated, so the individual has more than two alleles for certain traits. An inverted duplication is known to occur in chromosome 15. The term inverted indicates that a segment runs in the direction opposite from normal. Children with this syndrome, called inv dup 15 syndrome,have poor muscle tone, mental disabilities, seizures, a curved spine, and autistic characteristics that include poor speech, hand flapping, and lack of eye contact

Causes of Gene Mutations

A gene mutation can be caused by a number of factors, including errors in DNA replication, a transposon, or an environmental mutagen. Mutations due to DNA replication errors are rare: They occur with a frequency of 1 in 100 million cell divisions on average in most eukaryotes. This average is low due to DNA polymerase, the enzyme that carries out replication (see Section 11.1) and proofreads the new strand against the old strand, detecting and correcting any mismatched pairs. Mutagens are environmental influences that cause mutations. Different forms of radiation, such as radioactivity, X-rays, ultraviolet (UV) light, and chemical mutagens, such as pesticides and compounds in cigarette smoke, may cause breaks or chemical changes in DNA. If mutagens bring about a mutation in the DNA in an individual's gametes, the offspring of that individual may be affected. If the mutation occurs in the individual's body cells, cancer may result. The overall rate of mutation is low, however, because DNA repair enzymes constantly monitor for any irregularity and remedy the problem. Transposons are specific DNA sequences that have the remarkable ability to move within and between chromosomes (Fig. 13.1). Their movement often disrupts genes, rendering them nonfunctional. These "jumping genes" have now been discovered in almost every group of organisms, including bacteria, plants, fruit flies, and humans.

Translocation

A translocation is the movement of a segment from one chromosome to another, nonhomologous chromosome or the exchange of segments between nonhomologous chromosomes (Fig. 13.5a). A person who has both of the involved chromosomes has the normal amount of genetic material and a normal phenotype, unless the translocation breaks an allele into two pieces or fuses two genes together. The person who inherits only one of the translocated chromosomes will have only one copy of certain alleles and three copies of other alleles. A genetic counselor begins to suspect a translocation has occurred when spontaneous abortions are commonplace and family members suffer from various syndromes. A special microscopic technique allows a technician to determine that a translocation has occurred. In 5% of Down syndrome cases, a translocation that occurred in a previous generation between chromosomes 21 and 14 is the cause. As long as the two chromosomes are inherited together, the individual is normal. But in future generations a person may inherit two normal copies of chromosome 21 and the abnormal chromosome 14 that contains a segment of chromosome 21. In these cases, Down syndrome is not related to parental age but instead tends to run in the family of either the father or the mother. Some forms of cancer are also associated with translocations. One example is called chronic myeloid leukemia (CML). This translocation was first discovered in the 1970s when new staining techniques revealed a translocation of a portion of chromosome 22 to chromosome 9. This translocated chromosome is commonly called a Philadelphia chromosome. Individuals with CML have a rapid growth of white blood cells (Fig. 13.5b), which often prevents the ability of the body to form red blood cells and reduces the effectiveness of the immune system. Another example is Alagille syndrome, caused by a translocation between chromosomes 2 and 20. This translocation also often produces a deletion on chromosome 20. The syndrome may also produce abnormalities of the eyes and internal organs. The symptoms of Alagille syndrome range from mild to severe, so some people may not be aware they have the syndrome. In Burkitt's lymphoma, translocations between chromosome 8 and either chromosomes 2, 14, or 22 disrupt a gene that is associated with regulating the cell cycle, resulting in the formation of very fast-growing tumors.

DNA Analysis

Analysis of DNA following PCR has improved over the years. At first, the entire genome was treated with restriction enzymes, and because each person has different restriction enzyme sites, this yielded a unique collection of DNA fragments of various sizes. During a process called gel electrophoresis, whereby an electrical current is used to force DNA through a porous gel material, these fragments were separated according to their size. Smaller fragments moved farther through the gel than larger fragments, resulting in a pattern of distinctive bands, called a DNA profile, or DNA fingerprint. Now, short tandem repeat (STR) profiling is a preferred method. STRs are short sequences of DNA bases that recur several times, as in GATAGATAGATA. STR profiling is advantageous because it doesn't require the use of restriction enzymes. Instead, PCR is used to amplify target sequences of DNA, which are fluorescently labeled. The PCR products are placed in an automated DNA sequencer. As the PCR products move through the sequencer, the fluorescent labels are picked up by a laser. A detector then records the length of each DNA fragment. The fragments are different lengths because each person has a specific number of repeats at a particular location on the chromosome (i.e., at each STR locus). That is, the greater the number of STRs at a locus, the longer the DNA fragment amplified by PCR. Individuals who are homozygotes will have a single fragment, and heterozygotes will have two fragments of different lengths (Fig. 12.3). The more STR loci employed, the more confident scientists can be of distinctive results for each person. DNA fingerprinting has many uses. Medically, it can identify the presence of a viral infection or a mutated gene that could predispose someone to cancer. In forensics, DNA fingerprinting using a single sperm can be enough to identify a suspected rapist, because the DNA is amplified by PCR. The fingerprinting technique can also be used to identify the parents of a child or identify the remains of someone who has died, such as a victim of a natural disaster. In the future, we will undoubtedly see more applications of recombinant DNA technology that will greatly enrich our lives and improve our health.

Genetically Modified Animals

Biotechnology techniques have been developed to insert genes into the eggs of animals. For example, many types of animal eggs have taken up the gene for bovine growth hormone (BGH). The procedure has been used to produce larger fish, cows, pigs, rabbits, and sheep. Transgenic pigs supply many transplant organs for humans, a process called xenotransplantation. Like plants, animals can be genetically modified to increase their value as food products. A new form of transgenic salmon (Fig. 12.8) contains genes from two other fish species; these genes produce a growth hormone that allows the salmon to grow quicker. Interestingly, these salmon are also engineered to be triploid females, which makes them sterile. The Aedes aegypti mosquito acts as a vector for several human diseases, including dengue fever and chikungunya. A transgenic form of the mosquito being released in Florida contains a genetic "kill switch," which produces proteins that kill the offspring, thus reducing the size of the population. Gene pharming, the use of transgenic farm animals to produce pharmaceuticals, is being pursued by a number of firms. Genes that code for therapeutic and diagnostic proteins are incorporated into an animal's DNA, and the proteins appear in the animal's milk. It is possible to produce not only drugs but also vaccines by this method. Transgenic mice are routinely used in medical research to study human diseases. For example, the allele that causes cystic fibrosis can be cloned and inserted into mouse embryonic stem cells. Occasionally, a mouse embryo homozygous for cystic fibrosis will result. This embryo develops into a mutant mouse that has a phenotype similar to that of a human with cystic fibrosis. New drugs for the treatment of cystic fibrosis can then be tested in such mice. A similar research animal is the OncoMouse, which carries genes for the development of cancer.

Genetically Modified Plants

Corn, potato, soybean, and cotton plants have been engineered to be resistant to either insect predation or commonly used herbicides (Fig. 12.7). Some corn and cotton plants have been developed that are both insect- and herbicide-resistant. In 2015, 94% of the soybeans and 89% of the corn planted in the United States had been genetically engineered. If crops are resistant to a broad-spectrum herbicide and weeds are not, then the herbicide can be used to kill the weeds. When herbicide-resistant plants are planted, weeds are easily controlled, less tillage is needed, and soil erosion is minimized. One of the main focuses of genetic engineering of plants has been the development of crops with improved qualities, especially improvements that reduce waste from food spoilage. For example, by knocking out a gene that causes browning in apples, a company called Okanagan Specialty Fruits produced the Arctic Apple, a genetically modified apple with an increased shelf life. Innate is a genetically modified potato in which a process called RNA interference turns off the expression of genes associated with bruising. Progress has also been made to increase the food quality of crops. Soybeans have been developed that mainly produce the monounsaturated fatty acid oleic acid, a change that may improve human health. Other types of crop plants are genetically engineered to increase their productivity. Leaves can be engineered to lose less water and take in more carbon dioxide. This type of modification helps a range of crops grow successfully in various climates, including those that are more likely to experience drought or have a higher average temperature than the plant's normal growing climate. Other single-gene modifications allow plants to produce various products, including human hormones, clotting factors, antibodies, and vaccines.

DNA Sequencing

DNA sequencing is a procedure used to determine the order of nucleotides in a segment of DNA, often within a specific gene. DNA sequencing allows researchers to identify specific alleles that are associated with a disease and thus facilitate the development of medicines or treatments. Information from DNA sequencing also serves as the foundation for the study of forensic biology and even contributes to our understanding of our evolutionary history. When DNA technology was in its inception in the early 1970s, this technique was performed manually using dye-terminator substances or radioactive tracer elements attached to each of the four nucleotides during DNA replication, then deciphering the results from a pattern on a gel plate. Modern-day sequencing involves attaching dyes to the nucleotides and detecting the different dyes via a laser in an automated sequencing machine, which shows the order of nucleotides on a computer screen. To begin sequencing a segment of DNA, many copies of the segment are made, or replicated, using a procedure called the polymerase chain reaction.

Gene Therapy

Gene therapy is the insertion of genetic material into human cells for the treatment of a disorder. It includes procedures that give a patient healthy genes to make up for faulty genes, as well as the use of genes to treat various other human illnesses, such as cardiovascular disease and cancer. Gene therapy can be ex vivo(outside the body) or in vivo (inside the body). Viruses genetically modified to be safe can be used to ferry a normal gene into cells (Fig. 13.13), and so can liposomes, which are microscopic globules that form when lipoproteins are put into a solution and are specially prepared to enclose the normal gene. On the other hand, sometimes the gene is injected directly into a particular region of the body. This section discusses examples of ex vivo gene therapy (the gene is inserted into cells that have been removed and then returned to the body) and in vivo gene therapy (the gene is delivered directly into the body).

Gene Expression in Eukaryotes

In bacteria, a single promoter serves several genes that make up a transcription unit, while in eukaryotes, each gene has its own promoter where RNA polymerase binds. Bacteria rely mostly on transcriptional control, but eukaryotes employ a variety of mechanisms to regulate gene expression. These mechanisms affect whether a gene is expressed, the speed with which it is expressed, and how long it is expressed. Some mechanisms of gene expression occur in the nucleus; others occur in the cytoplasm (Fig. 11.20). In the nucleus, chromatin condensation, DNA transcription, and mRNA processing all play a role in determining which genes are expressed in a particular cell type. In the cytoplasm, mRNA translation into a polypeptide at the ribosomes can occur right away or be delayed. The mRNA can last a long time or be destroyed immediately, and the same holds true for a protein. These mechanisms control the quantity of gene product and/or how long it is active. Chromatin Condensation Eukaryotes utilize chromatin condensation as a way to keep genes turned on or off. The more tightly chromatin is compacted, the less often genes within it are expressed. Darkly staining portions of chromatin, called heterochromatin, represent tightly compacted, inactive chromatin. A dramatic example of this is the Barr body in mammalian females. Females have a small, darkly staining mass of condensed chromatin adhering to the inner edge of the nuclear envelope. This structure is an inactive X chromosome. How do we know that Barr bodies are inactive X chromosomes that are not producing gene product? Suppose 50% of the cells in a female have one X chromosome active, and 50% have the other X chromosome active. Wouldn't the body of a heterozygous female be a mosaic, with "patches" of genetically different cells? This is exactly what happens. For example, human females who are heterozygous for an X-linked recessive form of ocular albinism have patches of pigmented and nonpigmented cells at the back of the eye. And women who are heterozygous for the hereditary absence of sweat glands have patches of skin lacking sweat glands. The female calico cat also provides dramatic support for a difference in X-inactivation in its cells When heterochromatin undergoes unpacking, it becomes euchromatin, a less compacted form of chromatin that contains active genes. You learned in Section 8.1 that, in eukaryotes, a nucleosome is a portion of DNA wrapped around a group of histone molecules. When DNA is transcribed, a chromatin remodeling complex pushes aside the histone portions of nucleosomes, so that transcription can begin (Fig. 11.22). In other words, even euchromatin needs further modification before transcription can begin. The presence of histones limits access to DNA, and euchromatin becomes genetically active when histones no longer bar access to DNA. Only then is it possible for a gene to be turned on and expressed in a eukaryotic cell.

Signaling Between Cells in Eukaryotes

In multicellular organisms, cells are constantly sending out chemical signals that influence the behavior of other cells. During animal development, these signals determine the specialized role a cell will play in the organism. Later, the signals help coordinate growth and day-to-day functions. Plant cells also signal each other, so that their responses to environmental stimuli, such as direct sunlight, are coordinated. Typically, cell signaling occurs because a chemical signal binds to a receptor protein in a target cell's plasma membrane. The signal causes the receptor protein to initiate a series of reactions within a signal transduction pathway. The end product of the pathway (not the signal) directly affects the metabolism of the cell. For example, growth is possible only if certain genes have been turned on by regulatory proteins. In Figure 11.27, a signaling cell secretes a chemical signal that binds to a specific receptor located in the receiving cell's plasma membrane. The binding activates a series of reactions within a signal transduction pathway. The last reaction activates a transcription activator that enhances the transcription of a specific gene. Transcription leads to the translation of mRNA and a protein product that, in this case, stimulates the cell cycle, so that growth occurs. A protein called Ras functions in signal transduction pathways that lead to the transcription of many genes, several of which promote the cell cycle. Ras is normally inactive, but the reception of a growth factor leads to its activation. If Ras is continually activated, cancer will develop because cell division will occur continuously.

DNA Microarrays

New technologies have made DNA testing easy and inexpensive. For example, it is now possible to place thousands of known disease-associated mutant alleles onto a DNA microarray, also called a gene chip—a small silicon chip containing many DNA samples, in this case, the mutant alleles (Fig. 13.10). Genomic DNA from the subject to be tested is labeled with a fluorescent dye, then added to the microarray. The spots on the microarray fluoresce if the DNA binds to the mutant alleles on the chip, indicating that the subject may have a particular disorder or is at risk of developing it later in life. An individual's complete genotype, including all the various mutations, is called a genetic profile. With the help of a genetic counselor, individuals can be educated about their genetic profile. It's possible that a person has or will have a genetic disorder caused by a single pair of alleles. However, polygenic traits are more common, and in these instances, a genetic profile can indicate an increased or decreased risk for a disorder. Risk information can be used to design a program of medical surveillance and to foster a lifestyle aimed at reducing the risk. For example, suppose an individual has mutations common to people with colon cancer. It will be helpful for him or her to have an annual colonoscopy, so that any abnormal growths can be detected and removed before they became invasive.

Variations in Base Sequence

Scientists studying the human genome found that many small regions of DNA vary among individuals. For instance, there may be a difference in a single base within a gene (Fig. 12.9a and b) or within an intergenic sequence (Fig. 12.9c). One surprising finding is that some individuals even have additional copies of some genes (Fig. 12.9d)! Many of these differences have no ill effects, but some may increase or decrease an individual's susceptibility to disease.

DNA Sequencing

Recent advances in the processes of DNA sequencing (see Section 12.1) have made it much more feasible economically to sequence the genome of an individual to detect specific mutations associated with a disease. Whereas a few years ago the cost of sequencing an individual's genome could be as high as $100,000, that cost has decreased to almost $1,000. This decrease has ushered in an era of personal genomics, sometimes also referred to as personalized medicine. There are several approaches to personal genomics. While it is possible to sequence the entire genome of an individual, it is often more practical to target specific genes and look for alleles that are known to increase the risk associated with a specific disease. This approach is called a genome-wide association study,and it has become more prevalent in the field of personalized medicine due to the increase in large genomic population studies. One of the more interesting possibilities arising from personal genomics is pharmacogenomics, or the selection of a drug based on information coming directly from an individual's genome. In many cases, such as cancer, certain alleles associated with a gene will respond more effectively to a specific class of drugs. Thus, knowledge of the allelic combination of a patient can prove to be very beneficial to the physician.

Recombinant DNA Technology

Recombinant DNA (rDNA) contains genes from two or more different sources (Fig. 12.1). To make rDNA, a researcher needs a vector, a piece of DNA that acts as a carrier for the foreign DNA. One common vector is a plasmid, which is a small accessory ring of DNA found in bacterial cells. Two enzymes are needed to introduce foreign DNA into plasmid DNA: (1) restriction enzymes that can cleave, or cut, DNA at specific places (for example, the restriction enzyme EcoRI always cuts DNA at the base sequence GAATTC) and (2) DNA ligase, which can seal the foreign DNA into an opening in a cut plasmid. If a plasmid is cut with EcoRI, this creates a gap into which a piece of foreign DNA can be placed if that piece ends in bases complementary to those exposed by the restriction enzyme. To ensure that the bases are complementary, it is necessary to cleave the foreign DNA with the same restriction enzyme. The overhanging bases at the ends of the two DNA molecules are called "sticky ends," because they can bind a piece of foreign DNA by complementary base pairing. Sticky ends facilitate the insertion of foreign DNA into vector DNA, a process very similar to the way puzzle pieces fit together. DNA ligase, the enzyme that functions in DNA replication to repair breaks in a double-stranded helix, seals the foreign piece of DNA into the plasmid. Molecular biologists often give the rDNA to bacterial cells, which readily take up recombinant plasmids if the cells have been treated to make them more permeable. Thereafter, as the bacteria replicate the plasmid, the gene is cloned. Cloned genes have many uses. A scientist may allow the genetically modified bacterial cells to express the cloned gene and retrieve the protein. Or copies of the cloned gene may be removed from the bacterial cells and then introduced into another organism, such as a corn plant, to produce a transgenic organism.

Genetically Modified Bacteria

Recombinant DNA technology is used to produce transgenic bacteria, grown in huge vats called bioreactors (Fig. 12.6a). The gene product from the bacteria is collected from the medium. Such biotechnology products include insulin, clotting factor VIII, human growth hormone, tissue plasminogen activator (t-PA), and hepatitis B vaccine. Transgenic bacteria have many other uses as well. Some have been produced to promote the health of plants. For example, bacteria that normally live on plants and encourage the formation of ice crystals have been changed from frost-plus to frost-minus strains. As a result, new crops, such as frost-resistant strawberries and oranges, have been developed. Bacteria can be selected for their ability to degrade a particular substance, and this ability can then be enhanced by genetic engineering. For instance, naturally occurring bacteria that eat oil have been genetically modified to do this even more efficiently and used in cleaning up beaches after oil spills (Fig. 12.6b). Further, these bacteria are given "suicide" genes, which cause them to self-destruct when the job has been accomplished.

Genome Comparisons

Researchers are comparing the human genome with the genomes of other species for clues to our evolutionary origins. One surprising discovery is that the genomes of all vertebrates are similar. Researchers were not surprised to find that the genes of chimpanzees and humans are over 90% alike, but they did not expect to find that the human genome is also an 85% match with that of a mouse. Scientists also discovered that we share a number of genes with much simpler organisms, including bacteria! As the genomes of more organisms are sequenced, genome comparisons will likely reveal never previously expected evolutionary relationships between organisms. Genome comparison studies are also providing insight into our evolutionary heritage. By comparing the genome of modern-day humans with those of some of our more recent ancestors, such as the Neandertals and Denisovans (see Section 19.6), we are developing a better understanding of how our species has evolved over time. For example, several studies have compared the genes on chromosome 22 in humans and chimpanzees. Among the many genes that differed in sequence were several of particular interest: a gene for proper speech development, several for hearing, and several for smell. The gene necessary for proper speech development is thought to have played an important role in human evolution. Recent genomic studies have suggested that a change in intergenic sequences between 1 and 6 million years ago may have fundamentally changed the way the brain processes speech. Changes in genes affecting hearing may have facilitated the use of language for communication between people. Changes in smell genes are a little more problematic, but it is believed that the olfaction genes may have affected dietary changes or sexual selection. It was a surprise to find that many of the other genes that were located and studied are known to cause human diseases if abnormal. Perhaps comparing genomes is a way of finding genes associated with human diseases.

Sequencing the Bases of the Human Genome

The Human Genome Project, completed in 2003, represented a 13-year effort to determine the sequence of the 3.2 billion base pairs in the human genome. The project involved both university and private laboratories around the world. It was made possible by the rapid development of innovative technologies to sequence segements of DNA. Over the project's 13-year span, DNA sequencers were constantly improving, and modern automated sequencers can analyze and sequence up to 120 million base pairs of DNA in a 24-hour period.

Gene Expression in Prokaryotes

The bacterium Escherichia coli lives in the human large intestine and can quickly adjust its production of enzymes according to what a person eats. If you drink a glass of milk, E. coli immediately begins to make three enzymes needed to metabolize lactose. The transcription of all three enzymes is under the control of one promoter, a short DNA sequence where RNA polymerase attaches. French microbiologists François Jacob and Jacques Monod called such a cluster of bacterial genes, along with the DNA sequences that control their transcription, an operon. They received a Nobel Prize in 1961 for their investigations because they were the first to show how gene expression is controlled—specifically, how the lac operon is controlled in lactose metabolism Jacob and Monod proposed that a regulatory gene located outside the operon codes for a repressor—a protein that, in the lac operon, normally binds to the operator, which lies next to the promoter. When the repressor is attached to the operator, transcription of the lactose-metabolizing genes does not take place because RNA polymerase is unable to bind to the promoter. The lac operon is normally turned off in this way because lactose is usually absent (Fig. 11.19a). What turns the operon on when lactose is present? Lactose binds to the repressor and it changes shape. Now the repressor is unable to bind to the operator and RNA polymerase is able to bind to the promoter (Fig. 11.19b). Transcription of the genes needed for lactose metabolism occurs. When lactose is present and glucose, the preferred sugar, is absent, a protein called CAP (not shown in Fig. 11.19) assists in the binding of RNA polymerase to the promoter. This further ensures that the lactose-metabolizing enzymes are transcribed when they are needed. Other bacterial operons, such as those that control amino acid synthesis, are usually turned on. For example, in the trp operon, the regulatory gene codes for a repressor that ordinarily is unable to attach to the operator. Therefore, the genes needed to make the amino acid tryptophan are ordinarily expressed. When tryptophan is present, it binds to the repressor. A change in shape activates the repressor and allows it to bind to the operator. Now the operon is turned off.

mRNA processing

The fruit fly gene DScam offers a dramatic example of alternative mRNA processing. As many as 38,000 different proteins can be produced according to which of this gene's exons are combined in mature mRNAs. Evidence suggests that these proteins provide each nerve cell with a unique identity as it communicates with other nerve cells in a fruit fly's brain. mRNA Translation The cytoplasm contains proteins that can control whether translation of mRNA takes place. For example, an initiation factor, known as IF-2, inhibits the start of protein synthesis when it is phosphorylated by a specific protein kinase. Environmental conditions can also delay translation. Red blood cells do not produce hemoglobin unless heme, an iron-containing group, is available. The longer an mRNA remains in the cytoplasm before it is broken down, the more of the gene product is produced. During maturation, mammalian red blood cells eject their nuclei, yet they continue to synthesize hemoglobin for several months. The necessary mRNAs must be able to persist the entire time. Differences in the length of the poly-A tail can determine how long a particular mRNA transcript remains active. Hormones may also affect the stability of certain mRNA transcripts. An mRNA called vitellin persists for 3 weeks, instead of 15 hours, if it is exposed to estrogen. Protein Activity Some proteins are not active immediately after synthesis. For example, insulin is a single, long polypeptide that folds into a three-dimensional structure. Only then is a sequence of about 30 amino acids enzymatically removed from the middle of the molecule. This leaves two polypeptide chains bonded together by disulfide (SS) bonds, and an active insulin molecule results (Fig. 11.26). This mechanism allows a protein's activity to be delayed until it is needed. Many proteins are short-lived in cells because they are degraded or destroyed. Cyclins, which are proteins involved in regulating the cell cycle, are destroyed by giant enzyme complexes called proteasomes when they are no longer needed. Proteasomes break down other proteins as well.

Proteomics and Bioinformatics

The known sequence of bases in the human genome predicts that the approximately 23,000 genes are translated into over 200,000 different proteins due to alternative mRNA splicing; collectively, all of the proteins produced by an organism's genome are referred to as a proteome. The field of proteomics explores the structure and function of these cellular proteins and examines how they interact to contribute to the production of traits. Because drugs tend to be proteins or small molecules that affect the behavior of proteins, proteomics is crucial in the development of new drugs for the treatment of disease. Computer modeling of the three-dimensional shapes of proteins is an important part of proteomics, since structure relates to function. Researchers may then use the models to predict which molecules will be effective drugs. For example, a compound that fits the active site of an enzyme without being converted into a product may be an effective drug to inhibit that enzyme. Bioinformatics is the application of computer technologies to the study of the genome and proteome. Genomics and proteomics produce raw data, and these fields depend on computer analysis to find significant patterns in the data. As a result of bioinformatics, scientists hope to find cause-and-effect relationships between an individual's overall genetic makeup and resulting genetic disorders. More than half of the human genome consists of uncharacterized sequences that contain no genes and have no known function. Bioinformatics might be used to find the function of these regions by correlating any sequence changes with resulting phenotypes, or it might be used to discover that some sequences are an evolutionary relic that once coded for a protein that we no longer need. New computational tools will most likely be needed in order to accomplish these goals.

Polymerase Chain Reaction

The polymerase chain reaction (PCR) can create billions of copies of a segment of DNA in a test tube in a matter of hours. PCR is very specific—it amplifies (makes copies of) a targeted DNA sequence, usually a few hundred bases in length. PCR requires the use of DNA polymerase, the enzyme that carries out DNA replication, and a supply of nucleotides for the new DNA strands. PCR involves three basic steps (Fig. 12.2), which occur repeatedly, usually for about 35 to 40 cycles: (1) a denaturation step at 95°C, where DNA is heated so that it becomes single-stranded; (2) an annealing step at a temperature usually between 50°C and 60°C, where a nucleotide primer hybridizes (or binds) to each of the single DNA strands; and (3) an extension step at 72°C, where a DNA polymerase adds complementary bases to each of the single DNA strands, creating double-stranded DNA. PCR is a chain reaction because the targeted DNA is repeatedly replicated, much in the same way natural DNA replication occurs, as long as the process continues. Figure 12.2 uses color to distinguish the old strand from the new DNA strand. Notice that the amount of DNA doubles with each replication cycle. Thus, if you start with only one copy of DNA, after one cycle you will have two copies, after two cycles four copies, and so on. PCR has been in use since its development in 1985 by Kary Banks Mullis. The process relies on the discovery of a temperature-insensitive (thermostable) DNA polymerase that was extracted from the bacterium Thermus aquaticus, which lives in hot springs. The enzyme can withstand the high temperature used to denature double-stranded DNA. This enzyme can survive the high temperatures of a PCR reaction, which accelerates the production of copies of the selected DNA segment. DNA amplified by PCR is often analyzed for various purposes. For example, mitochondrial DNA base sequences have been used to decipher the evolutionary history of human populations. Because so little DNA is required for PCR to be effective, it is commonly used as a forensic method for analyzing DNA found at crime scenes—only a drop of semen, a flake of skin, or the root of a single hair is necessary!

Ultrasound

Ultrasound images help doctors evaluate fetal anatomy. An ultrasound probe scans the mother's abdomen, and a transducer transmits high-frequency sound waves, which are transformed into a picture on a video screen. This picture shows the fetus inside the uterus. Ultrasound can be used to determine a fetus's age and size, as well as the presence of more than one fetus. Also, some chromosomal abnormalities, such as Down syndrome, Edwards syndrome (three copies of chromosome 18), and Patau syndrome (three copies of chromosome 13), cause anatomical abnormalities during fetal development that may be detected by ultrasound by the twentieth week of pregnancy. For this reason, a routine ultrasound at this time is considered an essential part of prenatal care. Page 230Many other conditions, such as spina bifida, can be diagnosed by an ultrasound. Spina bifida results when the backbone fails to close properly around the spinal cord during the first month of pregnancy. Surgery to close a newborn's spine in such a case is generally performed within 24 hours after birth.

Testing the Egg

Unlike males, who produce four sperm cells following meiosis, meiosis in females results in the formation of a single egg and at least two nonfunctional cells called polar bodies. Polar bodies, which later disintegrate, receive very little cytoplasm, but they do receive a haploid number of chromosomes and thus can be useful for genetic diagnosis. When a woman is heterozygous for a recessive genetic disorder, about half the polar bodies receive the mutated allele, and in these instances the egg receives the normal allele. Therefore, if a polar body tests positive for a mutated allele, the egg probably received the normal allele (Fig. 13.12). Only normal eggs are then used for IVF. Even if the sperm should happen to carry the mutation, the zygote will, at worst, be heterozygous. But the phenotype will appear normal. If gene therapy becomes routine in the future, it's possible that an egg will be given genes that control traits desired by the parents, such as musical or athletic ability, prior to IVF. Such genetic manipulation, called eugenics, carries many ethical concerns.

Chromosomal Mutations

We have just reviewed the consequences of mutation at the level of the gene, but it is also possible for events, such as nondisjunction and mutation, to cause changes in the number of each chromosome in the cells or the structure of individual chromosomes. In humans, only a few variations in chromosome number, such as Down syndrome, Turner syndrome, and Klinefelter syndrome, are typically seen. These are usually caused by nondisjunction events during meiosis (see Section 9.4). Changes in chromosome structure, or chromosomal mutations, are much more common in the population. Syndromes that result from changes in chromosome structure are due to the breakage of chromosomes and their failure to reunite properly. Various environmental agents—radiation, certain organic chemicals, and even viruses—can cause chromosomes to break apart. Ordinarily, when breaks occur in chromosomes, the segments reunite to give the same sequence of genes. But their failure to do so results in one of several types of mutations: deletion, duplication, translocation, or inversion. Chromosomal mutations can occur during meiosis, and if the offspring inherits the abnormal chromosome, a syndrome may result.

Family Pedigrees

A pedigree is a chart of a family's history with regard to a particular genetic trait. In a pedigree, males are designated by squares and females by circles. Shaded circles and squares represent individuals expressing the genetic trait (often a disorder). A line between a square and a circle represents a union. A vertical line going downward leads directly to a single child; if there are more children, they are placed off a horizontal line. To identify specific individuals in a pedigree, generations are often numbered down the left side, and individuals are then identified from left to right. For example, in Figure 10.9, the male at the top is identified as individual I-2. The analysis of a pedigree is useful for genetic counselors. Once a pattern of inheritance has been established, the counselor can then determine the chances that any child born to the couple will have the abnormal phenotype.

One-Trait Inheritance

After ensuring that his pea plants were true-breeding—for example, that his plants with green pods always produced green-pod offspring and his yellow-pod plants always produced yellow-pod offspring—Mendel was ready to perform a cross between these two strains. Mendel called the original parents the P generation, the first-generation offspring the F1 (for filial) generation, and the second-generation offspring the F2 generation. The diagram in Figure 10.3representing Mendel's F1 cross is called a Punnett square (named after Reginald Punnett, an early-twentieth-century geneticist). In a Punnett square, all possible types of sperm are lined up vertically, and all possible types of eggs are lined up horizontally, or vice versa, so that every possible combination of gametes the offspring may inherit occurs within the square. As Figure 10.3 shows, when Mendel crossed green-pod plants with yellow-pod plants, all the F1 offspring resembled the green-pod parent. This did not mean that the other characteristic, yellow pods, had disappeared permanently. Notice that when Mendel allowed the F1 plants to self-pollinate, three-fourths of the F2 generation produced green pods and one-fourth produced yellow pods, a 3:1 ratio. Therefore, the F1 plants had been able to pass on a factor for the yellow color—it didn't just disappear. Mendel recognized that the F1 plants produced green pods because the green color was dominant over the yellow color. This type of cross is commonly called a monohybrid cross, since it involves two individuals who have two different alleles (they are heterozygous, Gg) for a single trait. Mendel's mathematical approach led him to interpret his results differently than previous breeders had done. He reasoned that a 3:1 ratio among the F2 offspring was possible only if (1) the F1 parents contained two separate copies of each hereditary factor, one dominant and the other recessive; (2) the factors separated when the gametes were formed, and each gamete carried only one copy of each factor; and (3) random fusion of all possible gametes occurred upon fertilization. Only in this way would the yellow color reoccur in the F2 generation.

Apoptosis

Apoptosis is often defined as programmed cell death because the cell progresses through a typical series of events that bring about its destruction (Fig. 8.10). The cell rounds up and loses contact with its neighbors. The nucleus fragments, and the plasma membrane develops blisters, called blebs. Finally, the cell breaks into fragments, which are engulfed by white blood cells and/or neighboring cells. Oddly enough, healthy cells routinely harbor the enzymes, called caspases, that bring about apoptosis. These enzymes are ordinarily held in check by inhibitors but are unleashed by either internal or external signals. Cell division and apoptosis are two opposing processes that keep the number of cells in the body at an appropriate level. In other words, cell division increases and apoptosis decreases the number of somatic cells (body cells). Both the cell cycle and apoptosis are normal parts of growth and development. An organism begins as a single cell that repeatedly undergoes the cell cycle to produce many cells, but eventually some cells must die for the organism to take shape. For example, when a tadpole becomes a frog, the tail disappears as apoptosis occurs. In humans, the fingers and toes of an embryo are at first webbed, but later they are freed from one another as a result of apoptosis. Apoptosis occurs all the time, particularly if an abnormal cell that could become cancerous appears. Death through apoptosis prevents a tumor from developing.

Characteristics of Cancer Cells

As we have seen, carcinogenesis is a gradual process that requires the accumulation of multiple mutations over time. Therefore, it may be decades before cancer develops and is diagnosed. Cancer cells have the following characteristics: 1. Cancer cells lack differentiation. Cancer cells lose their specialization and do not contribute to the functioning of a body part. A cancer cell does not look like a differentiated epithelial, muscle, nervous, or connective tissue cell; instead, it looks distinctly abnormal (Fig. 8.14). As mentioned, normal cells can enter the cell cycle about 70 times, and then they die. Cancer cells can enter the cell cycle repeatedly; in this way, they are immortal. 2. Cancer cells have abnormal nuclei. The nuclei of cancer cells are enlarged and may contain an abnormal number of chromosomes. The chromosomes are also abnormal; some parts may be duplicated, or some may be deleted. In addition, gene amplification (extra copies of specific genes) is seen much more frequently than in normal cells. 3. Cancer cells do not undergo apoptosis. Ordinarily, cells with damaged DNA undergo apoptosis, preventing tumors from developing. Cancer cells, however, do not respond normally to the signals to initiate apoptosis, and thus they continue to divide. 4. Cancer cells form tumors. Normal cells anchor themselves to a substratum and/or adhere to their neighbors. Then, they exhibit contact inhibition and stop dividing. Cancer cells, on the other hand, have lost all restraint; they pile on top of one another and grow in multiple layers, forming a tumor. They have a reduced need for growth factors, and they no longer respond to inhibitory signals. As cancer develops, the most aggressive cell becomes the dominant cell of the tumor. 5. Cancer cells undergo metastasis and promote angiogenesis. A benign tumor is usually contained within a capsule and therefore cannot invade adjacent tissue. Additional changes may cause cancer cells to produce enzymes that allow tumor cells to escape the capsule and invade nearby tissues. The tumor is then called a malignant tumor, meaning that it is invasive and may spread. Cells from a malignant tumor may travel through the blood or lymph to start new tumors elsewhere within the body. This process is known as metastasis (Fig. 8.15).

Other Cell Cycle Genes Associated with Cancer

BRCA1 and BRCA2 In 1990, DNA studies of large families in which females tended to develop breast cancer identified the first allele associated with that disease. Scientists named that gene breast cancer 1, or BRCA1. Other studies found that some forms of breast cancer were due to a faulty allele of another gene, called BRCA2. Both alleles are part of mutant tumor suppressor genes that are inherited in an autosomal recessive manner. If one mutated allele is inherited from either parent, a mutation in the other allele is required before the predisposition to cancer is increased. Because the first mutated gene is inherited, it is present in all cells of the body, and then cancer is more likely wherever the second mutation occurs, for example, in the breast or ovaries. RB Gene The RB gene is also a tumor suppressor gene. It takes its name from its association with an eye tumor called a retinoblastoma, which first appears as a white mass in the retina. A tumor in one eye is most common because it takes mutations in both alleles before cancer can develop. Children who inherit a mutated allele are more likely to have tumors in both eyes. Page 139RET Gene An abnormal allele of the RET gene, which predisposes a person to thyroid cancer, can be passed from parent to child. RET is a proto-oncogene known to be inherited in an autosomal dominant manner—only one mutated allele is needed to increase a predisposition to cancer. The remaining mutations necessary for thyroid cancer to develop are acquired (not inherited).

Chargaff's Rules

Before Erwin Chargaff began his work, it was known that DNA contains four different types of nucleotides, which contain different nitrogen-containing bases (Fig. 11.2). The bases adenine (A) and guanine (G) are purines with a double ring, and the bases thymine (T) and cytosine (C) are pyrimidines with a single ring. With the development of new chemical techniques in the 1940s, Chargaff decided to analyze in detail the base content of DNA in various species. In contrast to accepted beliefs, Chargaff found that each species has its own percentages of each type of nucleotide. For example, in a human cell, 31% of bases are adenine; 31% are thymine; 19% are guanine; and 19% are cytosine. In all the species Chargaff studied, the amount of A always equaled the amount of T, and the amount of G always equaled the amount of C. These relationships are called Chargaff's rules: - The amounts of A, T, G, and C in DNA varies from species to species. - In each species, the amounts of A and T are equal (A = T), as are the amounts of G and C (G = C). Chargaff's data suggest that DNA has a means to be stable, in that A can pair only with T and G can pair only with C. His data also show that DNA can be variable as required for the genetic material. Today, we know that the paired bases may occur in any order and the amount of variability in their sequences is overwhelming. For example, suppose a chromosome contains 140 million base pairs. Since any of the four possible nucleotide pairs can be present at each pair location, the total number of possible nucleotide pair sequences is 4140 × 106, or 4140,000,000.

Cancer Treatment

Cancer treatments either remove the tumor or interfere with the cancer cells' ability to reproduce. For many solid tumors, removal by surgery is often the first line of treatment. When the cancer is detected at an early stage, surgery may be sufficient to cure the patient by removing all cancerous cells. Because cancer cells are rapidly dividing, they are susceptible to radiation therapy and chemotherapy. The goal of radiation is to kill cancer cells within a specific tumor by directing high-energy beams at the tumor. DNA is damaged to the point that replication can no longer occur, and the cancer cells undergo apoptosis. Chemotherapy is a way to kill cancer cells that have spread throughout the body. Like radiation, most chemotherapeutic drugs lead to the death of cells by damaging their DNA or interfering with DNA synthesis. Others interfere with the functioning of the mitotic spindle. The drug vinblastine, first obtained from a flowering plant called a periwinkle, prevents the spindle from forming. Taxol, extracted from the bark of the Pacific yew tree, prevents the spindle from functioning as it should. Unfortunately, radiation and chemotherapy often damage cells other than cancer cells, leading to side effects, such as nausea and hair loss. Immunotherapy treatment involves using the patient's own immune system cells (see Section 26.3) to target cancer cells for destruction. These treatments involve identifying distinct differences between cancer cells and the normal cells of the body. In many cases, these treatments target slight differences in the composition of proteins in the plasma membranes of cancer cells and normal cells. Research in these areas is focusing on the development of monoclonal antibodies that act specifically on cancer cells and vaccines that prime the immune system to identify and destroy specific cancer cells.

Carcinogenesis

Carcinogenesis is the development of cancer. Figure 8.11 illustrates the development of colon cancer, a form of cancer that has been studied in detail. This figure shows that a single abnormal cell begins the process toward the development of a tumor. Along the way, the most aggressive cell becomes the dominant cell of the tumor. As additional mutations occur, the tumor cells release a growth factor, which causes neighboring blood vessels to branch into the cancerous tissue, a process called angiogenesis. Additional mutations allow cancer cells to produce enzymes that degrade the basement membrane and invade underlying tissues. Cancer cells are motile—able to travel through the blood or lymphatic vessels to other parts of the body—where they start distant tumors, a process called metastasis. Cells that are already highly specialized, such as nerve cells and cardiac muscle cells, seldom become cancer cells because they rarely divide. Carcinogenesis is more likely to begin in cells that have the capacity to enter the cell cycle. Fibroblasts and cells lining the cavities of the lungs, liver, uterus, and kidneys are able to divide when stimulated to do so. Adult stem cells continue to divide throughout life. These include blood-forming cells in the bone marrow and basal cells of the skin and digestive tract. Continuous division of these cells is required because blood cells live only a short while, and the cells that line the intestines and the cells that form the outer layer of the skin are continually sloughed off.

M (Mitotic) Phase

Cell division occurs during the M phase, which encompasses both division of the nucleus and division of the cytoplasm. The type of nuclear division associated with the cell cycle is called mitosis, which accounts for why this stage is called the M phase. During mitosis, the duplicated nuclear contents of the parent cell are distributed equally to the daughter cells. As a result of mitosis, the daughter nuclei are identical to the parent cell and to each other—they all have the same number and kinds of chromosomes. Recall that, during the S phase, the DNA of each chromosome is replicated to produce a duplicated chromosome that contains two identical sister chromatids, which remain attached at the centromere Each chromatid is a single DNA double helix containing the same sequence of base pairs as the original chromosome. During mitosis, the sister chromatids of each chromosome separate and are now called daughter chromosomes. Because each original chromosome goes through the same process of DNA replication followed by separation of the sister chromatids, the daughter nuclei produced by mitosis are genetically identical to each other and to the parent nucleus. Thus, if the parent nucleus has four chromosomes, each daughter nucleus also has four chromosomes of exactly the same type. One way to keep track of the number of chromosomes in drawings is to count the number of centromeres, because every chromosome has a centromere.

Replication of DNA

Cells need to make identical copies of themselves for the growth and repair of tissues. Before division occurs, each new cell requires an exact copy of the parent cell's DNA, so that it can pass on its genetic information to the next generation of cells. DNA replication refers to the process of making an identical copy of a DNA molecule. DNA replication occurs during the S phase of the cell cycle (see Fig. 8.3). During DNA replication, the two DNA strands, which are held together by hydrogen bonds, are separated and each old strand of the parent molecule serves as a template for a new strand in a daughter molecule (Fig. 11.6). This process is referred to as semiconservative, since one of the two old strands is conserved, or present, in each daughter molecule. To begin replication, the DNA double helix must separate and unwind. This is accomplished by breaking the hydrogen bonds between the nucleotides, then unwinding the helix structure using an enzyme called helicase. At this point, new nucleotides are added to the parental template strand. Nucleotides, ever present in the nucleus, will complementary base-pair onto the now single-stranded parental strand. The addition of the new strand is completed using an enzyme complex called DNA polymerase. The daughter strand is synthesized by DNA polymerase in a 5′-3′ direction, as shown in Figure 11.6. Any breaks in the sugar-phosphate backbone are sealed by the enzyme DNA ligase. In Figure 11.6, the backbones of the parent molecule (original double strand) are blue. Following replication, the daughter molecules each have a green backbone (new strand) and a blue backbone (old strand). A daughter DNA double helix has the same sequence of base pairs as the parent DNA double helix had. Complementary base pairing has allowed this sequence to be maintained. In eukaryotes, DNA replication begins at numerous sites, called origins of replication, along the length of the chromosome. At each origin of replication, a replication fork forms, allowing replication to proceed in both directions. Around each replication fork, a "replication bubble" forms. It is within the replication bubble that the process of DNA replication occurs. Replication proceeds along each strand in opposite directions until the entire double helix is copied. (Fig. 11.7). Although eukaryotes replicate their DNA at a fairly slow rate—500 to 5,000 base pairs per minute—there are many individual origins of replication throughout the DNA molecule. Therefore, eukaryotic cells complete the replication of the diploid amount of DNA (in humans, over 3 billion base pairs) in a matter of hours!

interphase phases

DNA replication occurs in the middle of interphase and serves as a way to divide interphase into three phases: G1, S, and G2. G1 is the phase before DNA replication, and G2 is the phase following DNA synthesis. Originally, G stood for "gap," but now that we know how metabolically active the cell is, it is better to think of G as standing for "growth." Protein synthesis is very much a part of these growth stages. 1. During G1, a cell doubles its organelles (such as mitochondria and ribosomes) and accumulates materials that will be used for DNA replication. At this point, the cell integrates internal and external signals and "decides" whether to continue with the cell cycle (see Section 8.3). Some cells, such as muscle cells, typically remain in interphase, and cell division is permanently arrested. These cells are said to have entered a G0 phase. If DNA damage occurs, many cells in the G0 phase can reenter the cell cycle and divide again to repair the damage. But a few cell types, such as nerve cells, almost never divide again once they have entered G0. 2. Following G1, the cell enters the S phase. The S stands for "synthesis," and certainly DNA synthesis is required for DNA replication. At the beginning of the S phase, each chromosome has one chromatid consisting of a single DNA double helix. At the end of this stage, each chromosome is composed of two sister chromatids, each having one double helix. The two chromatids of each chromosome remain attached at the centromere. DNA replication produces the duplicated chromosomes. 3. The G2 phase extends from the completion of DNA replication to the onset of mitosis. During this stage, the cell synthesizes the proteins that will be needed for cell division, such as the protein found in microtubules. The microtubules, part of the cytoskeleton, play an important role in cell division.

Genotype versus Phenotype

Different combinations of alleles for a trait can give an organism the same outward appearance. For instance, a recessive allele in humans (a) causes albinism. However, if you are AA or Aa and not aa, you have normal pigmentation. For this reason, it is necessary to distinguish between the alleles present in an organism and the appearance of that organism. The word genotype refers to the combination of alleles in a cell or organism. Genotype may be indicated by letters or by short, descriptive phrases. When an organism has two identical alleles for a trait, it is termed homozygous. A person who is homozygous dominant for normal pigmentation possesses two dominant alleles (AA). All gametes from this individual will contain an allele for normal pigmentation (A). Likewise, all gametes produced by a homozygous recessive parent (aa) contain an allele for albinism (a). Individuals who have two different alleles (Aa), are said to be heterozygous. The word phenotype refers to the physical appearance of the individual. An organism's phenotype is mostly determined by its genotype. The homozygous dominant (AA) individual and the heterozygous (Aa) individual both show the dominant phenotype and have normal pigmentation, while the homozygous recessive individual (aa) shows the recessive phenotype of albinism (Table 10.1).

Mendel's Experimental Procedure

Gregor Mendel was an Austrian monk who, after performing a series of ingenious experiments in the 1860s, developed several important laws on patterns of inheritance (Fig. 10.1a). Mendel studied science and mathematics at the University of Vienna, and at the time of his research in genetics, he was a substitute natural science teacher at a local high school. His background in mathematics prompted him to use a statistical basis for his breeding experiments. He prepared for his experiments carefully and conducted preliminary studies with various animals and plants. When Mendel began his work, most plant and animal breeders acknowledged that both sexes contribute equally to a new individual. However, they were unable to account for the presence of variations (or differences) among the members of a family, generation after generation. Mendel's models of heredity account for such variations. In addition, Mendel's models are compatible with the theory of evolution, which states that various combinations of traits are tested by the environment, and those combinations that lead to reproductive success are the ones that are passed on. Mendel's experimental organism was the garden pea, Pisum sativum. The garden pea was a good choice, since it was easy to cultivate and had a short generation time. And although peas normally self-pollinate (pollen goes only to the same flower), they could be cross-pollinated by hand. Many varieties of peas were available, and Mendel initially grew 22 of them. For his experiments, he chose 7 varieties that produced easily identifiable differences (Fig. 10.1b). When these varieties self-pollinated, they were true-breeding—meaning that the offspring were like the parent plants and like each other. In contrast to his predecessors, Mendel studied the inheritance of relatively simple and easily detected traits, such as seed shape, seed color, and flower color, and he observed no intermediate characteristics among the offspring. Figure 10.2 shows Mendel's procedure. As Mendel followed the inheritance of individual traits, he kept careful records. He used his understanding of the mathematical laws of probability to interpret his results and to arrive at a theory that has since been supported by innumerable experiments. This theory is called the particulate theory of inheritance, because it is based on the existence of minute particles we now call genes. Inheritance involves the reshuffling of the same genes from generation to generation.

The Watson and Crick Model

In 1951, James Watson, having just earned a Ph.D., began an internship at the University of Cambridge, England. There, he met Francis Crick, a British physicist who was interested in molecular structures. Together, they set out to determine the structure of DNA and to build a model that would explain how DNA varies from species to species, replicates, stores information, and undergoes mutation. Based on the available data, they knew the following: 1. DNA is a polymer of four types of nucleotides with the bases adenine (A), guanine (G), cytosine (C), and thymine (T). 2. Based on Chargaff's rules, A = T and G = C. 3. Based on Franklin's X-ray diffraction photograph, DNA is a double helix with a repeating pattern. Using these data, Watson and Crick built a model of DNA out of wire and tin (Fig. 11.4). The model showed that the deoxyribose sugar-phosphate molecules are bonded to one another to make up the sides of a twisted ladder. The Page 188nitrogenous bases make up the rungs of the ladder—they project into the middle and hydrogen bond with bases on the other strand. Indeed, the pairing of A with T and G with C—called complementary base pairing—results in rungs of a consistent width, as elucidated by the X-ray diffraction data. DNA is a double-stranded molecule that wraps around itself to form a double helix. The two strands are antiparallel and run in opposite directions, as best seen in Figure 11.5a. Notice that in the double helix, each strand has a 5′ end where a free appears and a 3′ end where a free —OH group appears. In Figure 11.5b, the carbon atoms in deoxyribose are numbered. The 5′ carbon has an attached group, and the 3′ carbon has an attached —OH group, which is circled and colored pink for easy recognition. The double-helix model of DNA permits the base pairs to be in any order, a necessity for genetic variability between species. Also, the model suggests that complementary base pairing may play a role in the replication of DNA. As Watson and Crick pointed out in their original paper, published in Nature in 1953, "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

Cytokinesis in Animal and Plant Cells

In most cells, cytokinesis follows mitosis. When mitosis occurs, but cytokinesis doesn't occur, the result is a multinucleated cell. Some organisms, such as fungi and slime molds, and certain structures in plants (such as the embryo sac) are multinucleated. Cytokinesis in Animal Cells In animal cells, a cleavage furrow, which is an indentation of the membrane between the two daughter nuclei, begins as anaphase draws to a close. The cleavage furrow deepens when a band of actin filaments, called the contractile ring, slowly forms a circular constriction between the two daughter cells. The action of the contractile ring can be likened to pulling a drawstring ever tighter around the middle of a balloon. A narrow bridge between the two cells is visible during telophase, and then the contractile ring continues to separate the cytoplasm until there are two independent daughter cells Cytokinesis in plant cells occurs by a process different from that seen in animal cells (Fig. 8.8). The rigid cell wall that surrounds plant cells does not permit cytokinesis by furrowing. Instead, cytokinesis in plant cells involves the building of new plasma membranes and cell walls between the daughter cells. Cytokinesis is apparent when a small, flattened disk appears between the two daughter plant cells. In electron micrographs, it is possible to see that the disk is composed of vesicles. The Golgi apparatus produces these vesicles, which move along microtubules to the region of the disk. As more vesicles arrive and fuse, a cell plate can be seen.

Incomplete Dominance

Incomplete dominance occurs when the heterozygote is intermediate between the two homozygotes. For example, when a curly-haired individual has children with a straight-haired individual, their children have wavy hair. When two wavy-haired persons have children, the expected phenotypic ratio among the offspring is 1:2:1—one curly-haired child to two with wavy hair to one with straight hair. We can explain incomplete dominance by assuming that the dominant allele encodes for a gene product that is not completely capable of masking the recessive allele. Another example of incomplete dominance in humans is familial hypercholesterolemia. This disease is due to a variation in the number of LDL-cholesterol receptor proteins in the plasma membrane. The number of receptors is related to the ability of the body to remove cholesterol from the blood. The fewer the number of receptors, the greater the concentration of cholesterol in the blood (Fig. 10.16). A person with two mutated alleles lacks LDL-cholesterol receptors. A person with only one mutated allele has half the normal number of receptors, and a person with two normal alleles has the usual number of receptors. People with the full number of receptors do not have familial hypercholesterolemia. When receptors are completely absent, excessive cholesterol is deposited in various places in the body, including under the skin. The presence of excessive cholesterol in the blood contributes to the development cardiovascular disease (see Section 23.3). Therefore, those with no LDL-cholesterol receptors die of cardiovascular disease as children. Individuals with half the number of receptors may die when young or after they have reached middle age.

From DNA to RNA to Protein

It's one thing to know that genes specify proteins and another to explain how they do it. Molecular genetics, which largely began when Watson and Crick discovered the structure of DNA in the 1950s, is able to explain exactly how genes control the building of specific types of proteins. Consider that, in eukaryotes, DNA resides in the nucleus but RNA is found both in the nucleus and in the cytoplasm, where protein synthesis occurs. This means that DNA must pass its genetic information to mRNA, which then actively participates in protein synthesis. The central dogma of molecular biology states that genetic information flows from DNA to RNA to protein Gene expression has occurred when a gene's product—the protein it specifies—is functioning in a cell. Specifically, gene expression requires two processes, called transcription and translation. In eukaryotes, transcription takes place in the nucleus, and translation takes place in the cytoplasm. During transcription, a portion of DNA serves as a template for mRNA formation. During translation, the sequence of mRNA bases (which are complementary to those in the template DNA) determines the sequence of amino acids in a polypeptide. So, in effect, molecular genetics tells us that genetic information lies in the sequence of the bases in DNA, which through mRNA determines the sequence of amino acids in a protein. tRNA assists mRNA during protein synthesis by bringing amino acids to the ribosomes. Proteins differ from one another by the sequence of their amino acids, and proteins determine the structure and function of cells and indeed the phenotype of the organism. It is important to note that the central dogma addresses the processing of genetic information for the purpose of producing a protein. However, scientists now recognize that there are segments of DNA that are expressed to form RNA molecules that have more of a regulatory role (such as micro-RNAs), and do not directly carry information that codes for proteins.

Cell Cycle Checkpoints

Just as a washing machine will not begin to agitate the load until the tub is full, the cell cycle has checkpoints that can delay the cell cycle until certain conditions are met. The cell cycle has many checkpoints, but we will consider only three: the G1, G2, and M checkpoints The G1 checkpoint is especially significant because, if the cell cycle passes this checkpoint, the cell is committed to divide. If the cell does not pass this checkpoint, it can enter G0, during which it performs its normal functions but does not divide. The proper growth signals, such as certain growth factors, must be present for a cell to pass the G1 checkpoint. Additionally, the integrity of the cell's DNA is checked. If the DNA is damaged, the protein p53 can stop the cycle at this checkpoint and initiate DNA repair. If repair is not possible, the protein can cause the cell to undergo programmed cell death, or apoptosis. The cell cycle halts momentarily at the G2 checkpoint until the cell verifies that DNA has replicated. This prevents the initiation of the M phase unless the chromosomes are duplicated. Also, if DNA is damaged, as from exposure to solar radiation or X-rays, arresting the cell cycle at this checkpoint allows time for the damage to be repaired, so that it is not passed on to daughter cells. Another cell cycle checkpoint occurs during the mitotic stage (M checkpoint). The cycle hesitates between metaphase and anaphase to make sure the chromosomes are properly attached to the spindle and will be distributed accurately to the daughter cells. The cell cycle does not continue until every chromosome is ready for the nuclear division process.

Crossing-Over

Meiosis not only reduces the chromosome number but also shuffles the genetic information between the homologous chromosomes. When a tetrad forms during synapsis, the nonsister chromatids may exchange genetic material, an event called crossing-over (Fig. 9.4a). Crossing-over occurs between nonsister chromatids of the two chromosomes in a tetrad. Notice in Figure 9.4b how one of the blue sister chromatids is exchanging information with the red nonsister chromatid of its tetrad. Crossing-over between the nonsister chromatids shuffles the alleles and serves as the way that meiosis brings about genetic recombination in the gametes. Recall that the homologous chromosomes carry the same genes for traits, such as the presence of freckles or a susceptibility to a disease, but the alleles may be different. Therefore, when the nonsister chromatids exchange genetic material, the sister chromatids then have a different combination of alleles, and the resulting gametes will be genetically different. Thus, in Figure 9.4b, even though two of the gametes will have the same chromosomes, these chromosomes may not have the same combination of alleles as before because of crossing-over. Crossing-over increases the diversity of the gametes and, therefore, of the offspring. At fertilization, new combinations of chromosomes can occur, and a zygote can have any one of a vast number of combinations of chromosomes. In humans, (223)2, or 70,368,744,000,000, chromosomally different zygotes are possible, even assuming no crossing-over.

Mitosis and Meiosis Occur at Different Times

Meiosis occurs only at certain times in the life cycle of sexually reproducing organisms, and only in specialized tissues. In humans, meiosis occurs only in the testes and ovaries, where it is involved in the production of gametes. The functions of meiosis are to provide gamete variation and to keep the chromosome number constant generation after generation. With fertilization, the full chromosome number is restored. Because unlike gametes fuse, fertilization introduces great genetic diversity into the offspring. Mitosis is much more common because it occurs in all tissues during embryonic development and during growth and repair. The function of mitosis is to keep the chromosome number constant in all the cells of the body, so that every cell has the same genetic material. These differences allow organisms, such as humans, to produce both the cells needed for the body to function and the cells used for reproduction

Overview of Meiosis

Meiosis results in four daughter cells because it consists of two divisions, called meiosis I and meiosis II (Fig. 9.3). Before meiosis I begins, each chromosome has duplicated and is composed of two sister chromatids. During meiosis I, the homologous chromosomes of each pair come together and line up side by side in an event called synapsis. Synapsis results in a tetrad, an association of four chromatids (two homologous chromosomes consisting of two chromatids each). The chromosomes of a tetrad stay in close proximity until they separate. Later in meiosis I, when the homologous chromosomes of each pair separate, one chromosome from each homologous pair goes to each daughter nucleus. No rules restrict which chromosome goes to which daughter nucleus. Therefore, all possible combinations of chromosomes may occur within the gametes. The cell does not reenter interphase between meiosis I and meiosis II, so there is no duplication of the chromosomes. Why? One of the roles of meiosis is to reduce the chromosome number; therefore, following meiosis I, the daughter nuclei each have half the number of chromosomes, but the chromosomes are still duplicated. The chromosomes are called dyads because each one is composed of two sister chromatids. During meiosis II, the sister chromatids of each dyad separate and become daughter chromosomes. The resulting four new daughter cells have the haploid number of chromosomes. If the parent cell has four chromosomes, then following meiosis each daughter cell has two chromosomes. (Remember that counting the centromeres tells you the number of chromosomes in a nucleus.) Recall that another purpose of meiosis is to produce genetically different gametes. Notice in Figure 9.3 that the daughter cells (on the right) do not have the same combinations of chromosomes as the original parent cell. Why not? The homologous chromosomes of each pair separated during meiosis I. Other chromosome combinations are possible in addition to those depicted. It's possible for the daughter cells to have chromosomes from only one parent (in our example, all red or all blue). The daughter cells from meiosis will complete either spermatogenesis (and become sperm) or oogenesis (and become eggs).

DNA and RNA Structure and Function

Mendel knew nothing about DNA. It took many years for investigators to come to the conclusion that Mendel's factors, now called genes, are on the chromosomes. Then, researchers wanted to show that DNA, not protein, is responsible for heredity. One experiment, by Alfred Hershey and Martha Chase, involved the use of a virus that attacks bacteria, such as E. coli (Fig. 11.1). A virus is composed of an outer capsid made of protein and an inner core of DNA. The use of radioactive tracers showed that the viral DNA, but not protein, enters bacteria and directs the formation of new viruses. By the early 1950s, investigators had learned that genes are composed of DNA and that mutated genes result in errors of metabolism. Therefore, DNA in some way must control the cell. While scientists did not yet know the actual structure of a DNA molecule, they did know its chemical components. The name deoxyribonucleic acid is derived from the chemical components of DNA's building blocks, the nucleotides. They also knew that DNA had the following characteristics: - Variability to account for differences in the wide variety of life on the planet. - The ability to replicate so that every cell gets a copy during cell division. - Storage of the information needed to control the cell. - The ability to change or mutate, to allow for the evolution of new species.

Two-Trait Inheritance

Mendel performed a second series of crosses in which he crossed true-breeding plants that differed in two traits (Fig. 10.6). For example, he crossed tall plants having green pods (TTGG) with short plants having yellow pods (ttgg). The F1 plants showed both dominant characteristics (tall with green pods). The question Mendel had was how these alleles would segregate in the F2 generation. There were two possible results that could occur in the F2 generation: 1. If the dominant factors (TG) always go together into the F1 gametes, and the recessive factors (tg) always stay together, then two phenotypes result among the F2 plants—tall plants with green pods and short plants with yellow pods. 2. If the four factors segregate into the F1 gametes independently, then four phenotypes result among the F2 plants—tall plants with green pods, tall plants with yellow pods, short plants with green pods, and short plants with yellow pods. Mendel observed four phenotypes among the F2 plants, supporting the second hypothesis. Therefore, Mendel formulated his second law of heredity—the law of independent assortment—which states the following: - Each pair of factors segregates (assorts) independently of the other pairs. - All possible combinations of factors can occur in the gametes. The type of cross illustrated in Figure 10.6 is also referred to as a dihybrid cross, since it is a cross for two traits, with each individual involved in the cross having two different alleles for each trait. Figure 10.6 illustrates that the expected phenotypic results of a dihybrid cross are always 9:3:3:1 when both parents are heterozygous for the two traits. However, it is important to note that ratios other than 9:3:3:1 are possible if the parents are not heterozygous for both traits.

One-Trait Testcross

Mendel's experimental use of simple dominant and recessive characteristics allowed him to test his interpretation of his crosses. To see if the F1 generation carries a recessive factor, Mendel crossed his F1 plants with green pods with true-breeding plants with yellow pods. He reasoned that half the offspring would produce green pods and half would produce yellow pods (Fig. 10.4 a). And, indeed, those were the results he obtained; therefore, his hypothesis that factors segregate when gametes are formed was supported. This is an example of a testcross. While the procedure has been largely replaced by more detailed genetic analyses (see Section 12.1), it is still used as an example of how to determine whether an individual with the dominant trait has two dominant factors for a particular trait. This is not possible to tell by observation because an individual can exhibit the dominant appearance while having only one dominant factor. Figure 10.4b shows that if the green-pod parent plant has two dominant factors, all the offspring will produce green pods.

Methemoglobinemia

Methemoglobinemia is a relatively harmless autosomal recessive disorder that results from an accumulation of methemoglobin, an alternative form of hemoglobin, in the blood. Since methemoglobin is blue instead of red, the skin of people with the disorder appears bluish-purple in color A persistent and determined physician finally solved the age-old mystery of what causes methemoglobinemia through blood tests and pedigree analysis of a family with the disorder, the Fugates of Troublesome Creek in Kentucky. On a hunch, the physician tested the Fugates for the enzyme diaphorase, which normally converts methemoglobin back to hemoglobin, and found that they indeed lacked the enzyme. Next, he treated a patient with the disorder in a simple but rather unconventional manner—by injecting a dye called methylene blue! The dye is a strong reducing agent capable of donating electrons to methemoglobin, converting it back into hemoglobin. The results were striking and immediate—the patient's skin quickly turned pink again.

Sex-Linked Inheritance

Normally, both males and females have 23 pairs of chromosomes; 22 pairs are called autosomes, and 1 pair is the sex chromosomes. The sex chromosomes are so named because they differ between the sexes. In humans, males have the sex chromosomes X and Y, and females have two X chromosomes The much shorter Y chromosome contains fewer than 200 genes, and most of these genes are concerned with sex differences between men and women. One of the genes on the Y chromosome, SRY, does not have a copy on the X chromosome. If the functional SRY gene is present, the individual becomes a male, and if it is absent, the individual becomes a female. Thus, female sex development is the "default setting." In contrast, the X chromosome is quite large and contains nearly 2,000 genes, most of which have nothing to do with the gender of the individual. By tradition, the term X-linked refers to such genes carried on the X chromosome. Examples of X-linked traits in humans include hemophilia and red-green color blindness. The Y chromosome does not carry these genes, which makes for an interesting inheritance pattern. It would be logical to suppose that a sex-linked trait is passed from father to son or from mother to daughter, but this is not always the case. For X-linked traits, a male always receives an X-linked allele from his mother, from whom he inherited an X chromosome. Because the X and Y chromosomes are not homologous, the Y chromosome from the father does not carry an allele for the trait. Usually, a sex-linked genetic disorder is recessive. Therefore, a female must receive two alleles, one from each parent, in order to develop the condition.

Linkage

Not long after Mendel's study of inheritance, investigators began to realize that many different types of alleles are on a single chromosome. A chromosome doesn't contain just one or two alleles; it contains a long series of alleles in a definite sequence. The sequence is fixed because each allele has its own locus on a chromosome. All the alleles on one chromosome form a linkage group because they tend to be inherited together. For example, Figure 10.22 shows some of the traits in the chromosome 19 linkage group of humans. When we do two-trait crosses (see Fig. 10.6), we are assuming that the alleles are on nonhomologous chromosomes and therefore are not linked. Alleles that are linked do not show independent assortment and therefore do not follow the typical Mendelian genotypic and phenotypic ratios. Historically, researchers used the differences between the expected Mendelian ratios and observed ratios of linked genes to construct genetic maps of the chromosomes. These maps indicated the relative distance between the genes on a chromosome. Today, genetic maps of chromosomes are obtained by direct sequencing of the chromosomes and other biochemical procedures.

Osteogenesis imperfecta

Osteogenesis imperfecta is an autosomal dominant genetic disorder that results in weakened, brittle bones. Although at least nine types of the disorder are known, most are linked to mutations in two genes necessary to the synthesis of a type I collagen—one of the most abundant proteins in the human body. Collagen has many roles, including providing strength and rigidity to bone and forming the framework for most of the body's tissues. Osteogenesis imperfecta leads to a defective type I collagen that causes the bones to be brittle and weak. Because the mutant collagen can cause structural defects even when combined with normal type I collagen, osteogenesis imperfecta is generally considered to be dominant. Osteogenesis imperfecta, which has an incidence of approximately 1 in 5,000 live births, affects all racial groups similarly and has been documented as long as 300 years ago. Some historians think that the Viking chieftain Ivar Ragnarsson, who was known as Ivar the Boneless and was often carried into battle on a shield, had this condition. In most cases, the diagnosis is made in young children who visit the emergency room frequently due to broken bones. Some children with the disorder have an unusual blue tint in the sclera, the white portion of the eye; reduced skin elasticity; weakened teeth; and occasionally heart valve abnormalities. Currently, the disorder is treatable with a number of drugs that help increase bone mass, but these drugs must be taken long-term.

Pleiotropy

Pleiotropy occurs when a single gene has more than one effect. Often, this leads to a syndrome, a group of symptoms that appear together and indicate the presence of a particular genetic mutation. For example, persons with Marfan syndrome have disproportionately long arms, legs, hands, and feet; a weakened aorta; and poor eyesight (Fig. 10.21). All of these characteristics are due to the production of abnormal connective tissue. Marfan syndrome has been linked to a mutated gene (FBN1) on chromosome 15 that ordinarily specifies a functional protein called fibrillin. This protein is essential for the formation of elastic fibers in connective tissue. Without the structural support of normal connective tissue, the aorta can burst, particularly if the person is engaged in a strenuous sport, such as volleyball or basketball. Flo Hyman may have been the best American woman volleyball player ever, but she fell to the floor and died when only 31 years old because her aorta gave way during a game. Now that coaches are aware of Marfan syndrome, they are on the lookout for it among very tall basketball players. Many other disorders, including sickle-cell disease and porphyria, are also examples of pleiotropic traits. Porphyria is caused by a chemical insufficiency in the production of hemoglobin, the pigment that makes red blood cells red. The symptoms of porphyria are photosensitivity, strong abdominal pain, port-wine-colored urine, and paralysis in the arms and legs. Many members of the British royal family in the late 1700s and early 1800s suffered from this disorder, which can lead to epileptic convulsions, bizarre behavior, and coma. The vampire legends are most likely also based on individuals with a specific form of porphyria.

Translation Has Three Phases

Polypeptide synthesis has three phases: initiation, an elongation cycle, and termination. Enzymes are required for each of the steps to occur, and energy is needed for the first two steps. Step 1: Initiation is the step that brings all of the translation components together - The small ribosomal subunit attaches to the mRNA in the vicinity of the start codon (AUG). - The anticodon of the initiator tRNA-methionine complex pairs with this codon. - The large ribosomal subunit joins to the small subunit. Step 2: During the elongation cycle, the polypeptide chain increases in length one amino acid at a time - The tRNA at the P site contains the growing peptide chain. - This tRNA passes its peptide to tRNA-amino acid at the A site. The tRNA at the P site enters the E site. - During translocation, the tRNA-peptide moves to the P site, the empty tRNA in the E site exits the ribosome, and the codon at the A site is ready for the next tRNA-amino acid. The complete cycle—complementary base pairing of new tRNA, transfer of the growing peptide chain, and translocation—is repeated at a rapid rate. The outgoing tRNA is recycled and can pick up another amino acid in the cytoplasm to take to the ribosome. Termination: Termination occurs when a stop codon (see Fig. 11.10) appears at the A site. Then, the polypeptide and the assembled components that carried out protein synthesis are separated from one another - A protein complex called a release factor binds to the stop codon and cleaves the polypeptide from the last tRNA. - The mRNA, ribosomes, and tRNA molecules can then be used for another round of translation.

Pedigrees for Sex-Linked Disorders

Recall that sons inherit X-linked recessive traits from their mother because their only X chromosome comes from their mother. More males than females have the disorder because recessive alleles on the X chromosome are always expressed in males—the Y chromosome lacks the allele. Females who have the condition inherited the allele from both their mother and their father, and all the sons of such a female will have the condition. If a male has an X-linked recessive condition, his daughters are all carriers even if his partner is normal. Therefore, X-linked recessive conditions often appear to pass from grandfather to grandson. Figure 10.25 lists other ways to recognize an X-linked recessive disorder. Still fewer traits are known to be X-linked dominant. If a condition is X-linked dominant, daughters of affected males have a 100% chance of having the condition. Females can pass an X-linked dominant allele to both sons and daughters. If a female is heterozygous and her partner is normal, each child has a 50% chance of escaping an X-linked dominant disorder, depending on which of the mother's X chromosomes is inherited. Still fewer genetic disorders involve genes carried on the Y chromosome. A counselor would recognize a Y-linked pattern of inheritance because Y-linked disorders are present only in males and are passed directly from a father to all sons but not to daughters.

Sickle-Cell Disease

Sickle-cell disease is an autosomal recessive disorder in which the red blood cells are not biconcave disks, like normal red blood cells, but are irregular in shape (Fig. 10.14). In fact, many are sickle-shaped. A single base change in the globin gene causes the hemoglobin in affected individuals to differ from normal hemoglobin by a single amino acid. The abnormal hemoglobin molecules stack up and form insoluble rods, and the red blood cells become sickle-shaped. Because sickle-shaped cells can't pass along narrow capillaries as well as disk-shaped cells can, they clog the vessels and break down. This is why persons with sickle-cell disease suffer from poor circulation, anemia, and low resistance to infection. Internal hemorrhaging leads to further complications, such as jaundice, episodic pain in the abdomen and joints, and damage to internal organs. Sickle-cell heterozygotes have normal blood cells but carry the sickle-cell trait. Most experts believe that persons that are heterozygous for the sickle-cell trait are generally healthy and do not need to restrict their physical activity. However, there occasionally may be problems if they experience dehydration or mild oxygen deprivation.

Two-Trait Testcross

The fruit fly Drosophila melanogaster, less than one-fifth the size of a housefly, is a favorite subject for genetic research because it has several mutant characteristics that are easily determined. The wild-type fly—the type you are most likely to find in nature—has long wings and a gray body, while some mutant flies have short (vestigial) wings and black (ebony) bodies. The key for a cross involving these traits is L = long wings, l = short wings, G = gray body, and g = black body. A two-trait testcross can be used to determine whether an individual is homozygous dominant or heterozygous for either of the two traits. Because it is not possible to determine the genotype of a long-winged, gray-bodied fly by inspection, this genotype may be represented as L__G__. In a two-trait testcross, an individual with the dominant phenotype for both traits is crossed with an individual with the recessive phenotype for both traits because this individual has a known genotype. For example, a long-winged, gray-bodied fly is crossed with a short-winged, black-bodied fly. The heterozygous parent fly (LIGg) forms four different types of gametes. The homozygous parent fly (llgg) forms only one type of gamete: As Figure 10.7 shows, all possible combinations of phenotypes occur among the offspring. This 1:1:1:1 phenotypic ratio shows that the L__G__ fly is heterozygous for both traits and has the genotype LlGg. A Punnett square can also be used to predict the chances of an offspring having a particular phenotype (Fig. 10.7). What are the chances of an offspring with long wings and a gray body? The chances are one in four, or 25%. What are the chances of an offspring with short wings and a gray body? The chances are also one in four, or 25%.

Mendel's Laws and Probability

The importance of Mendel's work is that it made a connection between observed patterns of inheritance and probability. When we use a Punnett square to calculate the results of genetic crosses, we assume that each gamete contains one allele for each trait (law of segregation) and that collectively the gametes have all possible combinations of alleles (law of independent assortment). Further, we assume that the male and female gametes combine at random—that is, all possible sperm have an equal chance to fertilize all possible eggs. Under these circumstances, we can use the rules of probability to calculate the expected phenotypic ratio. The rule of multiplication says that the chance of two (or more) independent events occurring together is the product of their chances of occurring separately. For example, the chance of getting tails when you toss a coin is ½. The chance of getting two tails when you toss two coins at once is ½ × ½ = ¼. Let's use the rule of multiplication to calculate the expected results in Figure 10.7. Because each allele pair separates independently, we can treat the cross as two separate one-trait crosses: The probability of the offspring's genotype being llgg is: 1/2 ll x 1/2 gg =1/4 llgg

The Genetic Code

The information contained in DNA and RNA is written in a chemical language different from that found in the protein specified by the DNA and RNA. The cell needs a way to translate one language into the other, and it uses the genetic code. But how can the four bases of RNA (A, C, G, U) provide enough combinations to code for the 20 amino acids found in proteins? If the code were a singlet code (one base standing for an amino acid), only four amino acids could be encoded. If the code were a doublet (any two bases standing for one amino acid), it still would not be possible to code for 20 amino acids. However, by using a triplet code, the four bases can supply 64 different combinations, far more than needed to code for 20 different amino acids. Each three-letter (three-base) unit of an mRNA molecule is called a codon, and it codes for a single amino acid (Fig. 11.10). Sixty-one codons correspond to a particular amino acid; the remaining three are stop codons that signal the end of a polypeptide. The codon that stands for the amino acid methionine is also used as a start codon that signals the initiation of translation. Most amino acids have more than one codon, which offers some protection against possibly harmful mutations that might change the sequence of the amino acids in a protein. Cracking the genetic code was no simple matter. Researchers performed a series of experiments in which they added artificial mRNA to a medium containing bacterial ribosomes and a mixture of amino acids. By comparing the bases in the mRNA with the resulting polypeptide, they were able to learn the code. For example, an mRNA with a sequence of repeating guanines (GGG′GGG′...) encoded a string of glycine amino acids, so the researchers concluded that the mRNA codon GGG specifies the amino acid glycine in a protein. The genetic code is considered to be almost universal for all life on Earth. This suggests an evolutionary aspect to the genetic code: that the code dates back to the very first organisms on Earth and that all life is related.

Protective Diet

The risk of some forms of cancer is up to 40% higher among obese men and women compared with people of normal weight. Thus, weight loss in these groups can reduce cancer risk. In addition, the following dietary guidelines are recommended: 1. Increase your consumption of foods that are rich in vitamins A and C (Fig. 8.16). Beta-carotene, a precursor of vitamin A, is found in dark green, leafy vegetables, carrots, and various fruits. Vitamin C is present in citrus fruits. These vitamins are called antioxidants, because in cells they prevent the formation of free radicals (organic ions having an unpaired electron), which can damage DNA. Vitamin C also prevents the conversion of nitrates and nitrites into carcinogenic nitrosamines in the digestive tract. 2. Avoid salt-cured or pickled foods because they may increase the risk of stomach and esophageal cancers. Smoked foods, such as ham and sausage, contain chemical carcinogens similar to those in tobacco smoke. Nitrites are sometimes added to processed meats (e.g., hot dogs and cold cuts) and other foods to protect them from spoilage; as mentioned, nitrites can be converted to cancer-causing nitrosamines in the digestive tract. 3. Include in the diet vegetables from the cabbage family, which includes cabbage, broccoli, brussels sprouts, kohlrabi, and cauliflower. Eating these vegetables may reduce the risk of gastrointestinal and respiratory tract cancers.

Protective Behaviors

To lower the risk of developing certain cancers, people are advised to avoid smoking, sunbathing, and excessive alcohol consumption. Cigarette smoking accounts for about 30% of all cancer deaths. Smoking is responsible for 90% of lung cancer cases among men and 80% among women. People who smoke two or more packs of cigarettes a day have lung cancer mortality rates 15 to 25 times greater than those of nonsmokers. Smokeless tobacco (chewing tobacco or snuff) increases the risk of cancers of the mouth, larynx, throat, and esophagus. Many skin cancers are sun-related. Sun exposure is a major factor in the development of the most dangerous type of skin cancer, melanoma, and the incidence of this cancer increases in people living near the equator. Similarly, excessive exposure to radon gas1 in homes increases the risk of lung cancer, especially in cigarette smokers. It is best to test your home and, if necessary, take the proper remedial actions. Cancers of the mouth, throat, esophagus, larynx, and liver occur more frequently among heavy drinkers, especially when accompanied by tobacco use (cigarettes, cigars, or chewing tobacco).

Mendel's Laws and Meiosis

Today, we are aware that Mendel's laws relate to the process of meiosis. In Figure 10.8, a human cell has two pairs of homologous chromosomes, recognized by length—one pair of homologues is short and the other is long. In these diagrams, the coloring is used to signify which parent the chromosome was inherited from; one homologue of each pair is the "paternal" chromosome, and the other is the "maternal" chromosome. When the homologues separate (segregate), each gamete receives one member from each pair. The homologues separate (assort) independently; it does not matter which member of each pair goes into which gamete. In the simplest of terms, a gamete in Figure 10.8 can receive one short and one long chromosome of either color. Therefore, all possible combinations of chromosomes are in the gametes.

Translation: An Overview

Translation is the second step by which gene expression leads to protein (polypeptide) synthesis. Translation requires several enzymes, mRNA, and the other two types of RNA: transfer RNA and ribosomal RNA. Transfer RNA Takes Amino Acids to the Ribosomes Each tRNA is a single-stranded nucleic acid that doubles back on itself such that complementary base pairing results in the cloverleaf-like shape shown in Figure 11.13. There is at least one tRNA molecule for each of the 20 amino acids found in proteins. The amino acid binds to one end of the molecule. The opposite end of the molecule contains an anticodon, a group of three bases that is complementary to a specific codon of mRNA. During translation, the order of codons in mRNA determines the order in which tRNAs bind at the ribosomes. When a tRNA-amino acid complex comes to a ribosome, its anticodon pairs with an mRNA codon. For example, if the codon is ACC, what is the anticodon, and what amino acid will be attached to the tRNA molecule? From Figure 11.10, we can determine this: Codon (mRNA)- ACC Anticodon (tRNA)- UGG Amino Acid (protein)- Threonine After translation is complete, a protein contains the sequence of amino acids originally specified by DNA. This is the genetic information that DNA stores and passes on to each cell during the cell cycle, then to the next generation of individuals. DNA's sequence of bases determines the proteins in a cell, and the proteins determine the function of each cell. Ribosomes Are the Site of Protein Synthesis Ribosomes are the small structural bodies where translation occurs. Ribosomes are composed of many proteins and several ribosomal RNAs (rRNAs). In eukaryotic cells, rRNA is produced in a nucleolus within the nucleus. There, it joins with proteins manufactured in the cytoplasm to form two ribosomal subunits, one large and one small. The subunits leave the nucleus and join together in the cytoplasm to form a ribosome just as protein synthesis begins. A ribosome has a binding site for mRNA as well as binding sites for two tRNA molecules at a time (Fig. 11.13). These binding sites facilitate complementary base pairing between tRNA anticodons and mRNA codons. The P binding site is for a tRNA attached to a peptide, and the A binding site is for a newly arrived tRNA attached to an amino acid. The E site is for tRNA molecules exiting the ribosome.

Meiosis Compared with Mitosis

We have now observed two different forms of cell division—mitosis (see Chapter 8) and meiosis. While there are some similarities, there are important differences between these processes (Fig. 9.6): - Meiosis requires two consecutive nuclear divisions, but mitosis requires only one nuclear division. - Meiosis produces four daughter nuclei, and there are four daughter cells following cytokinesis. Mitosis followed by cytokinesis results in two daughter cells. - Following meiosis, the four daughter cells are haploid, having half the number of chromosomes of the parent cell. Following mitosis, the daughter cells have the same number of chromosomes as the parent cell. - Following meiosis, the daughter cells are genetically different from each other and the parent cell. Following mitosis, the daughter cells are genetically identical to each other and to the parent cell.

Chromatin to Chromosomes

When a eukaryotic cell is not undergoing cell division, the DNA and associated proteins have the appearance of thin threads called chromatin. Closer examination reveals that chromatin is periodically wound around a core of eight protein molecules, so that it looks like beads on a string. The protein molecules are histones, and each bead is called a nucleosome. Chromatin normally adopts a zigzag structure, and then it is folded into loops for further compaction. This looped chromatin more easily fits within the nucleus. Just before cell division occurs, the chromatin condenses multiple times into large loops, which produces highly compacted chromosomes. The chromosomes are often 10,000 times more compact that the chromatin, which also allows them to be viewed with a light microscope. Prior to cell division, the chromosomes are duplicated. A duplicated chromosome is composed of two identical halves, called sister chromatids, held together at a constricted region called a centromere. Each sister chromatid contains an identical DNA double helix.

Proto-Oncogenes Become Oncogenes

When proto-oncogenes mutate, they become cancer-causing genes called oncogenes. Proto-oncogenes promote the cell cycle, and oncogenes accelerate the cell cycle. Therefore, a mutation that causes a proto-oncogene to become an oncogene is a "gain of function" mutation. A growth factor is a chemical signal that activates a cell-signaling pathway by bringing about phosphorylation of a signaling protein (Fig. 8.12a). The cell-signaling pathway then activates numerous proteins, many of which promote the cell cycle (Fig. 8.12b). Some proto-oncogenes code for a growth factor or for a receptor protein that receives a growth factor. When these proto-oncogenes become oncogenes, receptor proteins are easy to activate and may even be stimulated by a growth factor produced by the receiving cell. For example, the RAS proto-oncogenes promote mitosis when a growth factor binds to a receptor. When RASproto-oncogenes become oncogenes, they promote mitosis even when growth factors are not present. RAS oncogenes are involved in 20-30% of human cancers.

Chromosomal Rearrangements

When the chromosomes of cancer cells become unstable, portions of the DNA double helix may be lost, duplicated, or scrambled. For example, a portion of a chromosome may break off and reattach to another chromosome. These events, called translocations (see Section 13.2), may lead to cancer, especially if they disrupt genes that regulate the cell cycle. A phenomenon called a Philadelphia chromosome is the result of a translocation between chromosomes 9 and 22 (Fig. 8.13). The BCR (breakpoint cluster region) and ABL genes are fused, creating an oncogene, called BCR-ABL, that promotes cell division. This translocation causes nearly 95% of cases of chronic myelogenous leukemia (CML), a cancer of the bone marrow. Recently, researchers have used a drug called imatinib (Gleevec) to successfully treat CML. This drug inhibits the activity of the protein coded for by BCR-ABL. Encouraged by this success, scientists are now seeking to develop similar drugs that can inhibit the products of other oncogenes. Although cancer is usually a somatic disease, meaning that it develops only in body cells, some individuals may inherit a predisposition for developing some forms of cancer.

Tumor Suppressor Genes Become Inactive

When tumor suppressor genes mutate, their products no longer inhibit the cell cycle or promote apoptosis. Therefore, these mutations can be called "loss of function" mutations (Fig. 8.12). The p53 tumor suppressor gene produces a protein that checks the DNA for damage before it proceeds through the G1 checkpoint. If there are breaks in the DNA, the cell is instructed to enter into G0 phase, and if these repairs cannot be repaired, the cell undergoes apoptosis. Failure of the p53 gene to perform this function allows cells with DNA damage to rapidly divide, potentially leading to cancer (see chapter opener). Cancer develops gradually; multiple mutations usually occur before a cell becomes cancerous. As you can imagine, the conversion of proto-oncogenes into oncogenes, coupled with the inactivation of tumor suppressor genes, causes the cell cycle to continue unabated, just as a car with a sticking gas pedal and faulty brakes soon careens out of control.

During mitosis

a spindle arises that will separate the sister chromatids of each duplicated chromosome. Once separated, the sister chromatids become daughter chromosomes. In this way, the resulting daughter nuclei are identical to each other and to the parent nucleus. In Figure 8.5, the daughter nuclei not only have four chromosomes but they each have one red short, one blue short, one red long, and one blue long, the same as the parent cell had. In other words, each daughter nucleus is genetically the same as the original parent nucleus. Mitosis is usually followed by division of the cytoplasm, or cytokinesis. Cytokinesis begins during telophase and continues after the nuclei have formed in the daughter cells. The cell cycle is now complete, and the daughter cells enter interphase, during which the cell will grow and DNA will replicate once again.