Bio Unit 4

Translocation Sydromes

Some forms of cancer are 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. 10.16), which often prevents the ability of the body to form red blood cells and reduces the effectiveness of the immune system.

new cells are produced only when needed to repair damaged tissues (T/F)

false

Mitosis _________ the chromosome number, whereas meiosis _________ the chromosome number of the daughter cells.

maintains; decreases

homologous pair

maternal and paternal pair of a chromosome

the structures composed of tubulin subunits, that assist with chromosome movement during the cell cycle are called ___

microtubules

Involved in growth and repair of tissues

mitosis

chromosomes are found in the ___ of each cell

nucleus

Inversion

occurs when a segment of a chromosome turns 180 degrees

Duplication

presence of a chromosome segment more than once on the same chromosome

which statement best describes apoptosis

programmed cell death

the mitotic phase during which spindle fibers attach to the chromosomes is called ___

prometaphase

the spindle reforms with each new cell having a haploid number of chromosomes during ___ of meiosis

prophase II

in humans, meiosis occurs in male ___ to produce ___ and female ___ to produce ___

testes; sperm; ovaries; eggs

In which of the following is genetic material moved between nonhomologous chromosomes?

translocation

if the standard number of chromosomes in an organism's cell is 12, how many chromosomes will be in cells produced by mitosis

12

a human cell will contain ___ meters of DNA within the nucleus

2

chromosomes are the genetic material in eukaryotic cells. humans have ___ different chromosomes. they have ___ total chromosomes because they have ___ copies of each

23; 46; 2

if the standard number of chromosomes in an organism's cell is 12, how many chromosomes will a parent cell have just before cell division (after interphase)?

24

how many haploid daughter cells are there after telophase II

4

If an organism has 40 homologous chromosome pairs, what would be the haploid number

40

how many chromosomes are in somatic cells of humans

46

Hormone

A hormone is a chemical substance produced in the body that controls and regulates the activity of certain cells or organs. Hormones are essential for every activity of life.

Klinefelter Syndrome

A male with Klinefelter syndrome has two or more X chromosomes in addition to a Y chromosome (Fig. 10.13b). The extra X chromosomes become Barr bodies. The approximate incidence for Klinefelter syndrome is 1 in 500 to 1,000 males.

Tumor Suppressor Genes Become Inactive

A mutation in a tumor suppressor gene is much like brake failure in a car; when the mechanism that slows down and stops cell division does not function, the cell cycle accelerates and does not halt. Researchers have identified about a half-dozen tumor suppressor genes. Among these are the RB and p53 genes that code for the RB and p53 proteins. The Nature of Science feature, "The G1 Checkpoint," in Section 9.1 discusses the function of these proteins in controlling the cell cycle. The RB tumor suppressor gene was discovered when the inherited condition retinoblastoma was being studied, but malfunctions of this gene have now been identified in many other cancers as well, including breast, prostate, and bladder cancers. The p53 gene turns on the expression of other genes that inhibit the cell cycle. The p53 protein can also stimulate apoptosis. It is estimated that over half of human cancers involve an abnormal or deleted p53 gene.

Control of the Cell Cycle

A signal is an agent that influences the activities of a cell. Growth factors are signaling proteins received at the plasma membrane. Even cells arrested in G0 will finish the cell cycle if stimulated to do so by growth factors. In general, signals ensure that the cell cycle stages follow one another in the normal sequence. The red stop signs in Figure 9.1 represent three checkpoints at which the cell cycle either stops or continues on, depending on the internal signals received. Researchers have identified a family of internal signaling proteins, called cyclins, that increase and decrease as the cell cycle continues. Specific cyclins must be present for the cell to proceed from the G1 stage to the S stage and from the G2 stage to the M stage.

Which of the following is not an aneuploid condition?

Alagille syndrome

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. 9.2). The cell rounds up, causing it to lose contact with its neighbors. The nucleus fragments, and the plasma membrane develops blisters. Finally, the cell fragments are engulfed by white blood cells and/or neighboring cells.

Which is not true of the cell cycle?

Apoptosis occurs frequently during the cell cycle.

Meiosis II and Gamete Formation

At the beginning of meiosis II, the two daughter cells contain the haploid number of chromosomes, or one chromosome from each homologous pair. Note that these chromosomes still consist of duplicated sister chromatids at this point. During metaphase II, the chromosomes align at the metaphase plate, but they do not align in homologous pairs, as in meiosis I, because only one chromosome of each homologous pair is present (Fig. 10.6). Thus, the alignment of the chromosomes at the metaphase plate is similar to what is observed during mitosis. During meiosis II, sister chromatids separate, becoming daughter chromosomes that are distributed to the daughter nuclei. Following meiosis II, there are four haploid daughter cells. During anaphase II, the sister chromatids separate, becoming daughter chromosomes that are not duplicated. These daughter chromosomes move toward the poles. At the end of telophase II and cytokinesis, there are four haploid cells. Because of crossing-over of chromatids during meiosis I, each gamete most likely contains chromosomes with a mixture of maternal and paternal genes.

Deletion

Breakage of a chromosome end

Which of the following is not characteristic of cancer cells?

Cancer cells undergo apoptosis.

Cell Differentiation

Cell Differentiation is the process of a cell changing from one type of cell to another type. The cell usually changes to a more specialized type

Cell Division

Cell Division is when two cells divide into two or more daughter cells with the same genetic material.

which of the following processes are a part of the cell cycle

Equal distribution of chromosomes between daughter cells Partitioning of cellular components between daughter cells Replication of a cells chromosomes

indicate the organisms that use mitosis as a form of cell division

Fungi Plants

The stage of interphase that begins after S and goes to the beginning of mitosis is called

G1

Cytokinesis in Animal Cells

In animal cells a cleavage furrow, which is an indentation of the membrane between the two daughter nuclei, forms just as anaphase draws to a close. By that time, the newly forming cells have received a share of the cytoplasmic organelles that duplicated during the previous interphase. 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 about the middle of a balloon. As the drawstring is pulled tight, the balloon constricts in the middle as the material on either side of the constriction gathers in folds. These folds are represented by the longitudinal lines in Figure 9.6.

The Prokaryotic Chromosome

In electron micrographs, the bacterial chromosome appears as an electron-dense, irregularly shaped region called the nucleoid (L. nucleus, "nucleus, kernel"; Gk. -eides, "like"), which is not enclosed by a membrane. When stretched out, the chromosome is seen to be a circular loop with a length that is up to about a thousand times the length of the cell. Special enzymes and proteins help coil the chromosome so that it will fit within the prokaryotic cell.

Apoptosis and Cell Division

In living systems, opposing events keep the body in balance and maintain homeostasis. Cell division and apoptosis are two opposing processes that keep the number of cells in the body at an appropriate level. Cell division increases and apoptosis decreases the number of somatic cells. Both are normal parts of growth and development. An organism begins as a single cell that repeatedly divides 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 a human embryo, the fingers and toes are at first webbed, but then they are usually freed from one another as a result of apoptosis.

Overview of Meiosis

In sexually reproducing organisms, meiosis (Gk. mio, "less"; -sis, "act or process of") is the type of nuclear division that reduces the chromosome number from the diploid (2n) number (Gk. diplos, "twofold") to the haploid (n) number (Gk. haplos, "single"). The diploid (2n) number refers to the total number of chromosomes, which exists in two sets. The haploid (n) number of chromosomes is half the diploid number, or a single set of chromosomes. In humans, meiosis reduces the diploid number of 46 chromosomes to the haploid number of 23 chromosomes. Gametes, or reproductive cells (in animals, these are the sperm and egg), usually have the haploid number of chromosomes. In sexual reproduction, haploid gametes, which are produced during meiosis, subsequently merge into a diploid cell called a zygote. In plants and animals, the zygote undergoes development to become an adult organism.

Prophase I

It is apparent during prophase I that nuclear division is about to occur, because a spindle forms as the centrosomes migrate away from one another (Fig. 10.5). The nuclear envelope fragments, and the nucleolus disappears. Stages of Meiosis I When diploid homologous chromosomes pair during meiosis I, crossing-over occurs, as represented by the exchange of color. Pairs of homologous chromosomes separate during meiosis I, and chromatids separate, becoming haploid daughter chromosomes. Following interkinesis, two cells begin meiosis II (see Fig. 10.6).

During this stage, daughter chromosomes are distributed to two daughter nuclei.

M(mitotic) stage

The nuclear division that reduces the chromosome number from diploid to haploid is called

Meiosis

Nondisjunction may occur, causing abnormal gametes to form.

Meiosis I Meiosis II

The phases of Meiosis

Meiosis consists of two unique, consecutive cell divisions, meiosis I and meiosis II. DNA is replicated in S phase of the cell cycle, prior to meiosis I but not meiosis II. Both meiosis I and meiosis II contain a prophase, metaphase, anaphase, and telophase.

Occurrence

Meiosis occurs only at certain times in the life cycle of sexually reproducing organisms. In humans, meiosis occurs only in the reproductive organs and produces the gametes. Mitosis is more common, because it occurs in all tissues during growth and repair.

which stage does the cell enter after completing interphase

Mitosis

Mitosis

Mitosis is a type of cell division, or duplication. The result is two daughter cells that have the same number and kind of chromosomes as the parent nucleus.

The Functions of Mitosis

Mitosis permits growth and repair. In both plants and animals, mitosis is required during development as a single cell develops into an individual. In plants, the individual could be a fern or daisy, while in animals, the individual could be a grasshopper or a human.

Origin of Cancer

Normal growth and maintenance of body tissues depend on a balance between signals that promote and inhibit cell division. When this balance is upset, conditions such as cancer may occur. Thus, cancer is usually caused by mutations affecting genes that directly or indirectly affect this balance, such as those shown in Figure 9.9. The following two types of genes are usually affected: Proto-oncogenes code for proteins that promote the cell cycle and prevent apoptosis. They are often likened to the gas pedal of a car, because they cause the cell cycle to speed up. Tumor suppressor genes code for proteins that inhibit the cell cycle and promote apoptosis. They are often likened to the brakes of a car, because they cause the cell cycle to go more slowly or even stop.

Comparing Prokaryotes and Eukaryotes

Prokaryotes (bacteria and archaea), protists (many algae and protozoans), and some fungi (yeasts) are single-celled. Cell division in single-celled organisms produces two new individuals: In multicellular fungi (molds and mushrooms), plants, and animals, cell division is part of the growth process. It produces the multicellular form we recognize as the mature organism. Cell division is also important in multicellular forms for renewal and repair:

Binary Fission

Prokaryotes reproduce asexually by binary fission. The process is termed binary fission because division (fission) produces two (binary) daughter cells that are identical to the original parent cell. Before division takes place, the cell enlarges, and after DNA replication occurs, there are two chromosomes. These chromosomes attach to a special plasma membrane site and separate by an elongation of the cell that pulls them apart. During this period, a new plasma membrane and cell wall develop and grow inward to divide the cell. When the cell is approximately twice its original length, the new cell wall and plasma membrane for each cell are complete (Fig. 9.12).

which events occur in the G2 phase of the cell cycle

Rapid cell growth Synthesis of proteins

A eukaryotic cell replicates its DNA during the ___ stage (or phase) of interphase

S

Spermatogenesis

The testes contain stem cells called spermatogonia. These cells keep the testes supplied with primary spermatocytes that undergo spermatogenesis, as described in Figure 10.9. Primary spermatocytes with 46 chromosomes undergo meiosis I to form two secondary spermatocytes, each with 23 duplicated chromosomes. Secondary spermatocytes undergo meiosis II to produce four spermatids with 23 daughter chromosomes. Spermatids then differentiate into viable sperm (spermatozoa). Upon sexual arousal, the sperm enter ducts and exit the penis upon ejaculation.

Which of the following statements is true?

Tumor suppressor gene products inhibit the cell cycle.

Proto-oncogenes Become Oncogenes

When mutations occur in proto-oncogenes, they become oncogenes, or cancer-causing genes as shown in Figure 9.10. Oncogenes are under constant stimulation and keep on promoting the cell cycle regardless of circumstances. For example, an oncogene may code for a faulty receptor in the stimulatory pathway such that the cell cycle is stimulated, even when no growth factor is present! Or an oncogene may either specify an abnormal protein product or produce abnormally high levels of a normal product that stimulate the cell cycle to begin or to go to completion. As a result, uncontrolled cell division may occur.

the indentation formed in the cell membrane at the end of anaphase is called ___

a cleavage furrow

An individual with jacobs syndrome is a ____

a male with one X and two Y chromosomes

klinefelter syndrome results from

a male with two or more X chromosomes

if cytokinesis does not happen at the end of the cell cycle, what can result

a multinucleated cell

a cell plate is ___

a new plasma membrane that divides the cell in half

select the organisms that can reproduce asexually through cell division

algae bacteria archaea

A barr body is

an inactive X chromosome

the structural and functional unit of life is the

cell

The repeating sequence of events in eukaryotes that involves cell growth and cell division and includes the stages G1, S, G2 and mitosis is called the __ ____

cell cycle

before meiosis, chromosomes are duplicated to form sister ___ during the ___ phase of interphase

chromatids; S

the diffuse, granular combination of DNA and proteins is called

chromatin

which of the following events occur during prophase 1 (meiosis)

chromatin condenses to chromosomes homologous chromosomes pair in synapsis to create tetrads nuclear membrane dissolves crossing-over occurs between homologues to exchange DNA

which of the following events occur during prophase II (meiosis)

chromatin condenses to chromosomes nuclear membranes dissolves spindle fibers form

arrange the steps of binary fission in order

chromosome is replicated chromosomes segregated to each side of the cell septum forms across the cell during cytokinesis two identical cells are formed

homologous chromosomes exchange genetic information in an event called ___ ___ during ____ 1

crossing over; prophase

the term life __ refers to all the reproductive events that occur from one generation to the next generation

cycle

the division of the cytoplasm into two daughter cells following mitosis is called

cytokinesis

a human body cell that contains the full number of 46 chromosomes is considered a ___ cell

diploid

cells that contain the full number of each type of chromosome are typically referred to as

diploid

Mitosis

division of chromosomes

The form of DNA that contains genes that are actively being transcribed is called

euchromatin

sperm and eggs are reproductive or sex cells that are collectively referred to as

gametes

cells produced through meiosis are called _____. they include ___ in males and ___ in females

gametes; sperm; egg

during sexual reproduction, haploid ____ fuse to become a diploid ____

gametes; zygote

mitosis and meiosis involve division of the ___ material

genetic

daughter cells produced by mitosis will be

genetically identical

select the functions of mitosis

growth repair

some fungi, plants and animals use cell division as a means for ___

growth and repair

what are the functions of mitosis in multicellular eukaryotic organisms such as plants and animals

growth, development and repair

are cells going through prophase II haploid or diploid

haploid

normal gametes will contain the ___ number of chromosomes

haploid

In humans, gametogenesis results in the formation of

haploid egg and sperm cells.

sister chromatids

identical copies of each chromosome

which of the following occurs during G1? check all that apply

increasing in size accumulation of materials for DNA synthesis Increasing number of organelles

Which of the following occurs at metaphase I of meiosis?

independent assortment

which process occurs in a sequence of two divisions

meiosis

Crossing-over occurs between

nonsister chromatids of a bivalent.

a women that has three X chromosomes has a condition referred to as

poly-X

The G2 phase of the cell cycle is also known as the

pre-mitotic phase

arrange the phases of mitosis in order

prophase metaphase anaphase telophase

which of the following is characteristic of the S phase of the eukaryotic cell cycle

synthesis of DNA

What is DNA

the material that carries genetic information from one generation to the next

A bivalent is

the paired homologous chromosomes.

Chromatin consists of both DNA and proteins (T/F)

true

sexually reproducing species produce both sex cells (gametes) and somatic (non-sex or body) cells

true

spindle fibers are used to align the chromosomes along the metaphase plate (T/F)

true

elephants have twenty pairs of p53 genes, which are ___ suppressor genes; humans only have one pair, making them more prone to cancer when compared to elephants

tumor

a person with only one X chromosome (and no Y) has ___ syndrome

turner

Process

During prophase I, bivalents form and crossing-over occurs. These events do not occur during mitosis. During metaphase I of meiosis, bivalents independently align at the metaphase plate. The paired chromosomes have a total of four chromatids each. During metaphase in mitosis, individual chromosomes align at the metaphase plate. They each have two chromatids. During anaphase I of meiosis, homologues of each bivalent separate, and duplicated chromosomes (with centromeres intact) move to opposite poles. During anaphase of mitosis, sister chromatids separate, becoming daughter chromosomes that move to opposite poles. The events of meiosis II are similar to those of mitosis, except that in meiosis II the nuclei contain the haploid number of chromosomes (Table 10.2). In mitosis, the original number of chromosomes is maintained. Meiosis II produces two daughter cells from each parent cell that completes meiosis I, for a total of four daughter cells. These daughter cells contain the same number of chromosomes as they did at the end of meiosis I.

Telophase

During telophase, the spindle disappears as new nuclear envelopes form around the daughter chromosomes. Each daughter nucleus contains the same number and kinds of chromosomes as the original parent cell. Remnants of the polar spindle fibers are still visible between the two nuclei. The chromosomes become more diffuse chromatin once again, and a nucleolus appears in each daughter nucleus. Division of the cytoplasm requires cytokinesis, which is discussed next.

S Stage

Following G1, the cell enters the S stage, when DNA synthesis, or replication, occurs. At the beginning of the S stage, each chromosome is composed of one DNA double helix. Following DNA replication, each chromosome is composed of two identical DNA double helix molecules. Each double helix is called a chromatid, and the two identical chromatids are referred to as sister chromatids. The sister chromatids remain attached until they are separated during mitosis.

M (Mitotic Stage)

Following interphase, the cell enters the M (for mitotic) stage. This cell division stage includes mitosis (nuclear division) and cytokinesis (division of the cytoplasm). During mitosis, daughter Page 149chromosomes are distributed by the mitotic spindle to two daughter nuclei. When division of the cytoplasm is complete, two daughter cells are present.

Interkinesis

Following telophase, the cells enter interkinesis, a short rest period prior to beginning the second nuclear division, meiosis II. The process of interkinesis is similar to interphase between mitotic divisions, except that DNA replication does not occur, because the chromosomes are already duplicated.

G2 Stage

Following the S stage, G2 is the stage from the completion of DNA replication to the onset of mitosis. During this stage, the cell synthesizes the proteins that will assist cell division. For example, it makes the proteins that form microtubules. Microtubules are used during the mitotic stage to form the mitotic spindle that is critical during M stage.

Homologous Pairs of Chromosomes

In diploid body cells, the chromosomes occur in pairs. Figure 10.1a, a pictorial display of human chromosomes, called a karyotype, shows the chromosomes arranged according to pairs. The members of each pair are called homologous chromosomes. Homologous chromosomes, or homologues (Gk. homologos, "agreeing, corresponding"), look alike; they have the same length and centromere position. When stained, homologues have a similar banding pattern, because they contain genes for the same traits in the same order in the same locations on both chromosomes in the homologous pair. But while homologous chromosomes have genes for the same traits, such as finger length, the DNA (deoxyribonucleic acid) sequence for the gene on one homologue may code for short fingers and the gene at the same location on the other homologue may code for long fingers. Alternative forms of a gene (as for long fingers and short fingers) are called alleles. The DNA sequences of alleles are highly similar, but they are different enough to produce alternative physical traits, such as long or short fingers.

Fate of Daughter Cells

In the plant life cycle (see Section 23.1), the daughter cells become haploid spores that germinate to become a haploid generation. This generation then produces the gametes by mitosis. In the animal life cycle, the daughter cells become the gametes, either sperm or eggs. The body cells of an animal normally contain the diploid number of chromosomes due to the fusion of sperm and egg during fertilization. If meiotic events go wrong, the gametes can contain the wrong number of chromosomes or altered chromosomes.

Prophase

It is apparent during prophase that nuclear division is about to occur, because chromatin has condensed and the chromosomes are visible. Recall that DNA replication occurred during interphase, and therefore the parental chromosomes are already duplicated and composed of two sister chromatids held together at a centromere. Counting the number of centromeres in diagrammatic drawings gives the number of chromosomes for the cell depicted. During prophase, the nucleolus disappears and the nuclear envelope fragments. The spindle begins to assemble as the two centrosomes migrate away from one another. In animal cells, an array of microtubules radiates toward the plasma membrane from the centrosomes. These structures are called asters. It is thought that asters brace the centrioles during later stages of cell division. Notice that the chromosomes have no particular orientation, because the spindle has not yet formed.

Why is it important to have specific signals that cause cell division and differentiation?

It is important to have specific signals that cause cell division and differentiation because the decisions of individual cells to enter the cell cycle, maintain competence, become quiescent, expand, differentiate, or die depends on cell-to-cell communication and on the perception of various signals. Examples of signals are hormones, nutrients, light, temperature, and internal positional and developmental cues. Signals ensure that the cell cycle states follow one another in the normal sequence. Researchers have identified a family of internal signaling proteins, called cyclins, that increase and decrease as the cell cycle continues. Specific cyclins must be present for the cell to proceed to different stages in the cell cycle. For example, in the video we wanted the plant cells down in the bottom of our cuttings to divide into new cells and then when they divide, differentiate into root tissues instead of stem tissues. And that can work because every single cell in the plant has the genes for any type of cell. We just need the right genes to get expressed and the process that is going to make that happen is by giving these some sort of internal or external queue that is going to trigger them to release hormones and those hormones are going to tell the cells to divide and then differentiate into root tissues.

Meiosis I compared to Mitosis

Meiosis I compared to mitosis. Why does meiosis produce daughter cells with half the number of chromosomes, whereas mitosis produces daughter cells with the same number of chromosomes as the parent cell? Compare metaphase I of meiosis to metaphase of mitosis. Only in metaphase I of meiosis are the homologous chromosomes paired at the metaphase plate. Members of homologous chromosome pairs separate during anaphase I, and therefore the daughter cells are haploid. The exchange of color between nonsister chromatids represents the crossing-over that occurred during meiosis I. The blue chromosomes were inherited from the paternal parent, and the red chromosomes were inherited from the maternal parent.

Aneuploidy

Monosomy occurs when an individual has only one of a particular type of chromosome when he or she should have two. Trisomy (2n + 1) occurs when an individual has three of a particular type of chromosome when he or she should have two. Both monosomy and trisomy are the result of nondisjunction during mitosis or meiosis. Primary nondisjunction occurs during meiosis I when both members of a homologous pair go into the same daughter cell (Fig. 10.11a). Secondary nondisjunction occurs during meiosis II when the sister chromatids fail to separate and both daughter chromosomes go into the same gamete (Fig. 10.11b). In animals, monosomies and trisomies of nonsex, or autosomal, chromosomes are generally lethal, but a trisomic individual is more likely to survive than a monosomic one. In humans, only three autosomal trisomic conditions are known to be viable beyond birth: trisomy 13, 18, and 21. Only trisomy 21 is viable beyond early childhood and is characterized by a distinctive set of physical abnormalities and intellectual disabilities. In comparison, sex chromosome aneuploids are better tolerated in animals and have a better chance of producing survivors.

Stem Cells

Researchers are learning to manipulate the production of various types of tissues from adult stem cells in the laboratory. If successful, these tissues could be used to cure illnesses. As discussed in the Nature of Science feature, "Reproductive and Therapeutic Cloning," therapeutic cloning, which is used to produce human tissues, can begin with either adult stem cells or embryonic stem cells. Embryonic stem cells can also be used for reproductive cloning, the production of a new individual.

In which phase of the cell cycle are the chromosomes replicated

S

sister chromatids are present during which phase of the cell cycle

S

which phases of the cell cycle make up interphase

S G1 G2

sister chromatids are created in the ___ phase of the cell cycle

S (synthesis)

At the end of this stage, each chromosome consists of two attached chromatids.

S stage

The Cell Cycle

The cell cycle is an ordered set of stages that takes place between the time a eukaryotic cell divides and the time the resulting daughter cells also divide. When a cell is going to divide, it grows larger, the number of organelles doubles, and the amount of DNA doubles as DNA replication occurs. The two portions of the cell cycle are interphase, which includes a number of stages, and the mitotic stage, when mitosis and cytokinesis occur.

Jacobs Syndrome

XYY males, termed Jacobs syndrome, can result only from nondisjunction during spermatogenesis. Among all live male births, the frequency of the XYY karyotype is about 1 in 1,000. Affected males are usually taller than average, suffer from persistent acne, and tend to have speech and reading problems, but they are fertile and may have children. Despite the extra Y chromosome, there is no difference in behavior between XYY and XY males.

alternate forms of a gene, like forms of a gene for finger length that cause the formation of long vs. short fingers are called

alleles

alternative forms of the same gene are called

alleles

___ is the mitotic phase during which sister chromatids move toward the poles of the spindle

anaphase

During which phase of meiosis do homologous chromosomes separate?

anaphase I

during which phase of meiosis will homologous chromosomes move to opposite poles

anaphase I

which phase of meiosis do sister chromatids separate and move to opposite poles

anaphase II

during which phases of cell division are sister chromatids pulled apart from forming daughter chromosomes

anaphase II, anaphase

binary fission is used by prokaryotes for

asexual reproduction

single-celled protists and fungi reproduce through ___

asexual reproduction

homologous chromosomes separate independently or in a random manner during independent

assortment

the radiation of microtubules towards the plasma membrane during mitosis are called ___

asters

splitting of a prokaryotic parent cell into two identical daughter cells is called ___ fission

binary

Which of the following is the term used to describe asexual reproduction in a single-celled organism?

binary fission

prokaryotic cells divide through a process called ____ _____

binary fission

a tetrad is known as a(n)

bivalent

At the metaphase plate during metaphase I of meiosis, there are

bivalents homologous pairs of chromosomes.

___ occurs when control of the cell cycle fails

cancer

what method is used by animals for growth and repair of their tissues

cell division

The G1 phase of the cell cycle is characterized by

cell growth

G1

cell growth and normal activities

all living organisms are composed of

cells

centromere

central region of a chromosome

the animal specific structures that assists with cell division are called ___

centrioles

sister chromatids are attached together at the

centromere

the main microtubule-organizing center of the cell is called the

centrosome

If a parent cell has 16 chromosomes, then each of the daughter cells following meiosis will have

chromosomes

Just prior to cell division, DNA condenses into rod-shaped structures called

chromosomes

a karyotype is a visual representation of

chromosomes

inside a dividing cell, what are the condensed molecules of DNA and proteins called

chromosomes

which of the following events occur during metaphase

chromosomes align at center of the cell

Kinetochore

chromosomes attached to spindle fibers

which of the following events occur during telophase

chromosomes uncoil to chromatin spindle fibers disappear nuclear membranes reform

the genetic material is tightly packed into ___ during cell division, but normally exits as loosely arranged ___

chromosomes; chromatin

Histones are involved in

compacting the DNA molecule.

the division of the cytoplasm is called ___

cytokinesis

during telophase II, which of the following happens

cytokinesis is completed, resulting in four haploid cells

normal cells are ____ and have two copies of each of the 23 chromosomes. cells produced through meiosis are ____ and only have a single copy of each chromosome

diploid; haploid

Cytokinesis

division of the cell/cytoplasm

when an individual inherits an extra copy of chromosome it is referred to as

down syndrome

At the metaphase plate during metaphase of mitosis, there are

duplicated chromosomes.

before a cell undergoes division, the DNA and associated proteins are located within ___ which appears like a tangled mass of thin threads

euchromatin

(T/F) A human cell in metaphase I will look identical to a cell in metaphase of mitosis

false

(T/F) crossing-over occurs between the two sister chromatids of a bivalent

false

(T/F) during metaphase II homologous pairs of chromosomes will align at the metaphase plate

false

all animal cells have the same number of chromosomes (T/F)

false

all of the cells of a sexually producing organism are diploid

false

The synaptonemal complex

forms during prophase I of meiosis.

an original diploid parent cell will produce how many daughter cells when meiosis is complete

four

the haploid number of chromosomes is

half the diploid number

the daughter cells produced during meiosis of a plant's life cycle will eventually germinate to become a ____ generation

haploid

in human males, spermatogenesis produces

haploid sperm

in a plant's life cycle daughter cells of meiosis become

haploid spores

cells are ____ at the beginning of meiosis II because they only have one of each homologous chromosome. however, each of the chromosomes still have identical ___ chromatid

haploid; sister

In contrast to a eukaryotic chromosome, a prokaryotic chromosome

has a single loop of DNA.

Paired chromosomes that have the same length and centromere position are called

homologous chromosomes

the maternal and paternal pair of each chromosome are called ___ ___

homologous chromosomes

_______ ___ of chromosomes are separated during meiosis 1 and two cells are formed

homologous pairs

which of the following events occurs during anaphase 1 (meiosis)

homologous pairs (tetrads) are pulled apart

in metaphase 1 of meiosis, ___ pairs line up in the middle of the cell. in metaphase of mitosis sister ___ line up in the middle of the cell

homologous; chromatids

in metaphase I of meiosis, ___ pairs line up in the middle of the cell. In ____ II of meiosis, sister _____ line up in the middle of the cell

homologous; metaphase; chromatids

another name for homologous chromosomes is

homologues

which of the following events occur during metaphase 1 (meiosis)

independent assortment occurs homologous chromosome pairs (tetrads) align at center of cell

homologous pairs align independently at the center of the cell during an event called ____ _____ during _____ 1. this creates a random mix of maternal and paternal ____ in the new cells

independent assortment; metaphase; chromosomes

Mitosis follows the G2 stage of ___ during the cell cycle

interphase

the G1, S, and G2 phases are collectively called

interphase

The diploid number of chromosomes

is the 2n number. is in a parent cell and therefore in the two daughter cells following mitosis. varies according to the particular organism. is present in most somatic cells. All of these are correct.*****

the protein complexes that develop on either side of the centromere are known as

kinetochores

gametes are formed in animals by a process of cell division called

meiosis

which type of cell division do tetrads occur

meiosis

which type of cell division does crossing-over occur

meiosis

which type of cell division produces haploid cells

meiosis

Involves pairing of duplicated homologous chromosomes

meiosis I

___ produces cells with half the number of original chromosomes. a normal human cell has ___ Chromosomes. the cells produced through meiosis have ___ chromosomes

meiosis; 46; 23

____ produces cells with half the number of original chromosomes. a normal human cell has ___ chromosomes. the cells produces through meiosis have ___ chromosomes

meiosis; 46; 23

during which phase of mitosis are animal cell chromosomes the same distance from both spindle poles

metaphase

during which phase(s) of cellular division are homologous pairs of chromosomes facing towards opposite poles

metaphase I

A parent cell with five duplicated chromosomes will produce daughter cells with five chromosomes consisting of one chromatid each.

mitosis

a fertilized egg divides by

mitosis

the type of cell division that is used in tissue repair is

mitosis

which type of cell division do homologous pairs stay together

mitosis

which type of cell division has a single round of division

mitosis

there are two types of cell division in eukaryotic cells. __ is used to make new identical cells. ___ is used to make sperm and eggs

mitosis; meiosis

___ creates identical diploid cells and ___ creates unique ____gametes

mitosis; meiosis; haploid

if an egg that is missing a chromosome is fertilized by a normal sperm, the offspring has inherited

monosomy

Translocation

movement of one chromosome segment to another chromosome

the failure of homologous chromosomes to separate during meiosis is called

nondisjunction

specifically, mitosis means

nuclear division

a cell in prophase could have which of the following characteristics

nuclear envelope fragments movement of centrosomes away from each other

The bacterial chromosome is contained within the region called the

nucleoid

a length of chromatic coiled around a histone "core," which gives it a bead-like appearance is called a

nucleosome

___ tell a cell to divide. ___ ____ genes tell a cell to stop dividing

oncogenes; tumor suppressor

prokaryotic cells have ___ chromosome

one

Nondisjunction during meiosis I of oogenesis will result in eggs that have

one too many chromosomes. one less than the normal number of chromosomes.

During which mitotic phases are duplicated chromosomes present?

only during metaphase at the metaphase plate

egg cells are formed by a process called

oogenesis

Polar bodies are formed during the process of

oogenesis.

G2

organelles duplicated

in humans, oogenesis occurs within the

ovaries

Phases of Mitosis

prophase prometaphase metaphase anaphase telophase

arrange the phases of meiosis I in order

prophase 1 metaphase 1 anaphase 1 telophase 1

arrange the phases of meiosis in order

prophase 1 metaphase 1 anaphase 1 telophase 1 prophase II metaphase II anaphase II telophase II

which of the following properly lists the sequential order of meiotic phases

prophase 1, metaphase 1, anaphase 1, telophase 1, prophase II, metaphse II, anaphase II, telophase II

the central purpose of meiosis is to

reduce the chromosome number from 2n to n

select the functions of mitosis

repair growth

which of the following events occur during interphase

replication of DNA (genes) cell growth

which of the following events occur during interphase (meiosis)

replication of DNA (sisters created) cell growth

what is the difference between how sex cells are created and how typical somatic cells are replicated

sex cells are created by meiosis and somatic cells by mitosis

___ ___ are separated during meiosis II and four new cells are formed

sister chromatids

what structures are attached to each other at a centromere

sister chromatids

which events occur during metaphase II (meiosis)

sister chromatids align at the center of cell

which events occur during anaphase II (meiosis)

sister chromatids are pulled apart

which of the following events occur during anaphase

sister chromatids are pulled apart

prior to cell division, every chromosome is duplicated to create identical ___ ___. At this point immediately before division, a human cell would have 92 total chromosomes. This allows each new cell to have the complete set of ____ chromosomes after division

sister chromatids; 46

the DNA is copied to create ____ chromatids during the ___ phase of the cell cycle

sister; S

the body cells (those that do NOT undergo meiosis to become sperm or egg) are called ___ cells

somatic

the male gametes are known as __ cells

sperm

If a male animal has a diploid number of 16, what will be the results of meiosis in this organism

sperm cells with 8 chromosomes

which events occur during telophase II (meiosis)

spindle fibers disappear chromosomes uncoil to chromatin nuclear membranes reform

which of the following events occur during telophase 1 (meiosis)

spindle fibers disappear chromosomes uncoil to chromatin nuclear membranes reform

which of the following events occur during prophase

spindle fibers form chromatin condenses to chromosomes nuclear membrane dissolves

the phase of mitosis that follows anaphase is

telophase

the spindle disappears and interkinesis follows ___ I of meiosis I

telophase

Which of these is paired incorrectly?

telophase—a resting phase between cell division cycles

the series of events that results in the duplication of a cells genetic material and division of the cell is called ___

the cell cycle

which of the following is true about a homologous chromosome pair

the contain genes for the same traits but different variations

this type of cloning uses stem cells to produce mature cell types

therapeutic

which is true of daughter cells produced by meiosis II

they are genetically variable they are haploid

how do cells produce new cells

they divide and create two new cells form a single parent cell

which statement is correct about daughter cells produced by mitosis

they will be genetically identical

if a human egg with 24 chromosomes is fertilized by a normal sperm the result is

trisomy

(T/F) two daughter cells produced by mitosis will have the same number of chromosomes as the original parent cell

true

At the metaphase plate during metaphase II of meiosis, there are

unpaired duplicated chromosomes.

the merging of two gametes result in a(n)

zygote

Poly-X Females

A poly-X female, sometimes called a superfemale, has more than two X chromosomes and, therefore, extra Barr bodies in the nucleus. Females with three X chromosomes have no distinctive phenotype aside from a tendency to be tall and thin. Although some have delayed motor and language development, as well as learning problems, most poly-X females do not have an intellectual disability. Some may have menstrual difficulties, but many menstruate regularly and are fertile. Children usually have a normal karyotype. The incidence for poly-X females is about 1 in 1,500 females.

Interphase

As Figure 9.1 shows, most of the cell cycle is spent in interphase. This is the time when a cell performs its usual functions, depending on its location in the body. The amount of time the cell takes for interphase varies widely. Embryonic cells complete the entire cell cycle in just a few hours. For adult mammalian cells, interphase lasts for about 20 hours, which is 90% of the cell cycle. In the past, interphase was known as the resting stage. However, today it is known that interphase is very busy, and that preparations are being made for mitosis. Interphase consists of three stages, referred to as G1, S, and G2.

Summarize what happened in the plant shoot cells when they received an effective treatment.

As I discussed earlier, our results showed that the last place treatment went to the cuttings that were soaked in water for 24 hours. They had the lowest amount of root growth on them. The treatment that came in first place was the willow cuttings and rooting powders. The reason that treatment worked and was successful was because they sent some chemical signals to the cells of the shoots to tell the cells to divide and tell them to differentiate into root cells. And they grew really fast because of that. Mitosis is when a cell splits into two new daughter cells. It duplicates its DNA and the new cells have the same pieces and genetic code. If we looked at them under the microscope, we would see lots of cells in the phases of mitosis like prophase, metaphase, anaphase and telophase because they are rapidly dividing down in the new roots that are growing. During prophase, a cell gets the idea that it is time to divide. During metaphase, all of the pieces align themselves and getting ready for the split so certain pieces need to be in the right position, and generally prepare the cell for mitotic division. The DNA lines up along a central axis and the centrioles send out specialized tubules that connect to the DNA. The DNA are then condensed into the centromere. The tubules connect to the centromere, not to the DNA. During Anaphase, separation begins. Half of the chromosomes are pulled to one side of the cell and half go the other way. When the chromosomes get to the side of the cell, it is time to move to telophase. During telophase, the division is finishing up. This is when the cell membrane closes in and splits the cell into two pieces, where you now have two separate cells each with half of the original DNA. The process of mitosis explains what happens to cells when they received an effective treatment and are able to start growing new roots.

Other Causes of Cancer

As mentioned previously, cancer develops when the delicate balance between promotion and inhibition of cell division is tilted toward uncontrolled cell division. Other mutations may occur within a cell that affect this balance. For example, while a mutation affecting the cell's DNA repair system will not immediately cause cancer, it leads to a much greater chance of a mutation occurring within a proto-oncogene or tumor suppressor gene. And in some cancer cells, mutation of the telomerase enzyme that regulates the length of telomeres or the ends of chromosomes, causes the telomeres to remain at a constant length. Because cells with shortened telomeres normally stop dividing, keeping the telomeres at a constant length allows the cancer cells to continue dividing over and over again.

Cytokinesis in Animal and Plant Cells

As mentioned previously, cytokinesis is division of the cytoplasm. Cytokinesis accompanies mitosis in most cells, but not all. When mitosis occurs but cytokinesis doesn't occur, the result is a multinucleated cell. For example, you will see in Section 27.1 that the embryo sac in flowering plants is multinucleated. Division of the cytoplasm begins in anaphase, continues in telophase, but does not reach completion until the following interphase begins. By the end of mitosis, each newly forming cell has received a share of the cytoplasmic organelles that duplicated during interphase. Cytokinesis proceeds differently in plant and animal cells because of differences in cell structure.

Chromosome Number

As we observed in Section 9.2, the DNA in the chromosomes of eukaryotes is associated with various proteins. When a eukaryotic cell is not undergoing division, the DNA and associated proteins are structured as euchromatin, which has the appearance of a tangled mass of thin threads. Before mitosis begins, chromatin becomes highly coiled and condensed, and it is easy to see the individual chromosomes. When the chromosomes are visible, it is possible to photograph and count them. Each species has a characteristic chromosome number (Table 9.1). This is the full, or diploid (2n), number (Gk. diplos, "twofold"; -eides, "like") of chromosomes that is found in all cells of the individual. The diploid number includes two chromosomes of each kind. Most somatic cells of animals are diploid. Half the diploid number, called the haploid (n) number (Gk. haplos, "simple, single"), contains only one chromosome of each kind. The gametes of animals (egg and sperm) are examples of haploid cells.

Significance of Genetic Variation

Asexual reproduction passes on exactly the same combination of chromosomes and genes. Asexual reproduction may be advantageous if the environment remains unchanged. However, if the environment changes, genetic variability among offspring introduced by sexual reproduction may be advantageous. Under the new conditions, some offspring may have a better chance of survival and reproductive success than others in a population. For example, suppose the ambient temperature were to rise due to climate change. This change in the environment could place demands on the physiology of an organism. For example, an animal with less fur, or reduced body fat, could have an advantage over other individuals of its generation.

Anaphase

At the start of anaphase the two sister chromatids of each duplicated chromosome separate at the centromere, giving rise to two daughter chromosomes. Daughter chromosomes, each with a centromere and single chromatid composed of a single double helix, appear to move toward opposite poles. Actually, the daughter chromosomes are being pulled to the opposite poles as the kinetochore spindle fibers disassemble at the region of the kinetochores. Even as the daughter chromosomes move toward the spindle poles, the poles themselves are moving farther apart, because the polar spindle fibers are sliding past one another. Microtubule-associated proteins, such as the motor molecules kinesin and dynein, are involved in the sliding process. Anaphase is the shortest phase of mitosis.

The Cell Cycle and Cancer

Cancer is a cellular growth disorder that occurs when cells divide uncontrollably. Although causes widely differ, most cancers are the result of accumulating mutations that ultimately cause a loss of control of the cell cycle. Although cancers vary greatly, they usually follow a common multistep progression (Fig. 9.8). Most cancers begin as an abnormal cell growth that is benign, or not cancerous, and usually does not grow larger. However, additional mutations may occur, causing the abnormal cells to fail to respond to inhibiting signals that control the cell cycle. When this occurs, the growth becomes malignant, meaning that it is cancerous and possesses the ability to spread.

Telophase I

Completion of telophase I is not necessary during meiosis. That is, the spindle disappears, but new nuclear envelopes need not form before the daughter cells proceed to meiosis II. Also, this phase may or may not be accompanied by cytokinesis, which is separation of the cytoplasm. Notice in Figure 10.5 that the cells have different chromosome combinations than the original parent cell (not all of the combinations are shown in Fig. 10.5). The cells exiting telophase I are also haploid compared to the diploid parent cell.

Anaphase I

During anaphase I, the homologues of each bivalent separate and move to opposite poles, but sister chromatids do not separate. This splitting of the homologous pair reduces the chromosome number from 2n to n. However, each chromosome still has two chromatids (Fig. 10.5).

Independent Assortment of Homologous Chromosomes

During independent assortment, the homologous chromosome pairs separate independently, or randomly. When homologues align at the metaphase plate, the maternal or paternal homologue may be oriented toward either pole. Figure 10.4 shows the possible chromosome orientations for a cell that contains only three pairs of homologous chromosomes. Once all possible alignments of independent assortment are considered for these three pairs, the result will be 23, or 8, combinations of maternal and paternal chromosomes in the resulting gametes from this cell, simply due to independent assortment of homologues.

Metaphase I

During metaphase I, the bivalents held together by chiasmata (see Fig. 10.3) have moved toward the metaphase plate (equator of the spindle). Metaphase I is characterized by a fully formed spindle and alignment of the bivalents at the metaphase plate. As in mitosis, kinetochores are seen, but the two kinetochores of a duplicated chromosome are attached to the same kinetochore spindle fiber.

Metaphase

During metaphase, the centromeres of chromosomes are now in alignment on a single plane at the center of the cell. The chromosomes usually appear as a straight line across the middle of the cell when viewed under a light microscope. An imaginary plane that is perpendicular and passes through this circle is called the metaphase plate. It indicates the future axis of cell division. Several non attached spindle fibers, called polar spindle fibers, reach beyond the metaphase plate and overlap. A cell cycle checkpoint, the M checkpoint, delays the start of anaphase until the kinetochores of each chromosome are attached properly to spindle fibers and the chromosomes are properly aligned along the metaphase plate.

Chromosome Duplication

During mitosis, a 2n nucleus divides to produce daughter nuclei that are also 2n. The dividing cell is called the parent cell, and the resulting cells are called the daughter cells. Before nuclear division takes place, DNA replicates, duplicating the chromosomes in the parent cell. This occurs during the S stage of interphase. Now each chromosome has two identical double helical molecules. Each double helix is a chromatid, and the two identical chromatids are called sister chromatids (Fig. 9.4). Sister chromatids are constricted and attached to each other at a region called the centromere. Protein complexes called kinetochores develop on either side of the centromere during cell division.

Prometaphase (Late Prophase)

During prometaphase preparations for sister chromatid separation are evident. Kinetochores appear on each side of the centromere, and these attach sister chromatids to the kinetochore spindle fibers. These fibers extend from the poles to the chromosomes, which will soon be located at the center of the spindle.

the phase of the cell cycle in which a cell performs final preparations for mitosis is called

G2

The cell synthesizes the proteins needed for cell division.

G2 stage

If a certain eukaryotic organism had 12 chromosomes in its somatic cells, what is this organism's haploid number

6

The Eukaryotic Chromosome

Cell biologists and geneticists have been able to construct detailed models of how chromosomes are organized. A eukaryotic chromosome contains a single double helix DNA molecule, but it is composed of more than 50% protein. Some of these proteins are concerned with DNA and RNA synthesis, but a large majority, termed histones, play primarily a structural role. A human cell contains at least 2 meters of DNA, yet all of this DNA is packed into a nucleus that is about 6 μm in diameter. The histones are responsible for packaging the DNA so that it can fit into such a small space. First, the DNA double helix is wound at intervals around a core of eight histone molecules (two copies each of H2A, H2B, H3, and H4), giving the appearance of a string of beads (Fig. 9.3a). Each bead is called a nucleosome, and the nucleosomes are said to be joined by "linker" DNA. This string is compacted by folding into a zigzag structure, further shortening the DNA strand (Fig. 9.3b). Histone H1 appears to mediate this coiling process. The fiber then loops back and forth into radial loops (Fig. 9.3c). This loosely coiled euchromatin represents the active chromatin containing genes that are being transcribed. The DNA of euchromatin may be accessed by RNA polymerase and other factors that are needed to promote transcription. In fact, recent research seems to indicate that regulating the level of compaction of the DNA is an important method of controlling gene expression in the cell.

G1 Stage

Cell biologists named the stage before DNA replication G1, and they named the stage after DNA replication G2. 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." During G1, the cell recovers from the previous division. The cell grows in size, increases the number of organelles (such as mitochondria and ribosomes), and accumulates materials that will be used for DNA synthesis. Otherwise, cells are constantly performing their normal daily functions during G1, including communicating with other cells, secreting substances, and carrying out cellular respiration.

Explain the processes of cell division and cell differentiation. How are they different and how are they related?

Cell division is when two cells divide into two or more daughter cells with the same genetic material. Depending on the type of cell there are two ways cells divide, mitosis and meiosis. Each of these two processes has special characteristics. In mitosis, a single cell divides into two cells that are replicas of each other and have the same number of chromosomes. In mitosis the daughter cells each have the same number of chromosomes and DNA as the parent cell. The daughter cells are called diploid cells, which have two complete sets of chromosomes. Since the daughter cells have the exact same copies of their parents cell's DNA, no genetic diversity is created through mitosis in normal healthy cells. This type of cell division is good for basic growth, repair, and maintenance. In meiosis, a cell divides into four cells that have half the number of chromosomes. During meiosis, a small portion of each chromosome breaks off and reattaches to another chromosome. This process is called crossing over or genetic recombination. Genetic recombination is the reason full siblings made from egg and sperm cells from the same two parents can look very differently from one another. Reducing the number of chromosomes by half is important for sexual reproduction and provides genetic diversity. Cell differentiation is the process of a cell changing from one cell type to another. Usually, the cell changes to a more specialized type. Differentiation occurs numerous times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types. The process of a cell differentiation starts with the fertilization of the female egg. As soon as the egg is fertilized, cell multiplication is initiated resulting in the formation of a sphere of cells known as the blastocyst. This sphere of cells attach to the uterine wall and continue to differentiate. As the blastocyst differentiates, it divides and specialize to form a zygote that attaches to the womb for nutrients. As it continues to multiply and increase in size, the differentiation process results in the formation of different organs. Cell division and cell differentiation are different because cell division involves the splitting of a cell in order to form new cells. Cell differentiation allows a cell to specialize to achieve a specific function. During cell differentiation there is dramatic changes in a cell's shape, size, membrane potential, metabolic activity and responsiveness to signals. During cell division, the cell divides but contains the same genetic material and is not dramatically changed otherwise. Cell division and cell differentiation are related because they both involve changes in cells. In addition, cells in multicellular organisms repeatedly divide to make more cells for growth and repair. Without cell division the surface area to volume ratio that contains the size of single cells would limit an organism's growth. Cell differentiation needs cell division to happen in order to change the cell function or phenotypic type.

Changes in Chromosome Structure

Changes in chromosome structure include deletions, duplications, translocations, and inversions of chromosome segments. A deletion occurs when an end of a chromosome breaks off or when two simultaneous breaks lead to the loss of an internal segment (Fig. 10.14a). Even when only one member of a pair of chromosomes is affected, a deletion often causes abnormalities.

S

Genetic material duplicated

Genetic Recombination

Crossing-over is an exchange of genetic material between nonsister chromatids of a bivalent during meiosis I. In humans, it is estimated that an average of two to three crossovers occur between the nonsister chromatids during meiosis. At synapsis, homologues line up side by side, and a nucleoprotein lattice appears between them (Fig. 10.3). This lattice holds the bivalent together in such a Page 170way that the DNA of the duplicated chromosomes of each homologue pair is aligned. This ensures that the genes contained on the nonsister chromatids are directly aligned. Now crossing-over may occur. As the lattice breaks down, homologues are temporarily held together by chiasmata (sing., chiasma), regions where the nonsister chromatids are attached due to DNA strand exchange and crossing-over. After exchange of genetic information between the nonsister chromatids, the homologues separate and are distributed to different daughter cells. To appreciate the significance of crossing-over, keep in mind that the members of a homologous pair can carry slightly different instructions, or alleles, for the same genetic traits. In the end, due to a swapping of genetic material during crossing-over, the chromatids held together by a centromere are no longer identical. Therefore, when the chromatids separate during meiosis II, some of the daughter cells receive daughter chromosomes with recombined alleles. Due to genetic recombination, the offspring have a different set of alleles, and therefore genes, than their parents. This increases the genetic variation of the offspring. See the Nature of Science feature, "Meiosis and the Parthenogenic Lizards," to learn how a special form of crossing-over can increase diversity during asexual reproduction.

Cytokinesis in Plant Cells

Cytokinesis is apparent when a small, flattened disk appears between the two daughter plant cells near the site where the metaphase plate once was. In electron micrographs, it is possible that the disk is at right angles to a set of microtubules that radiate outward from the forming nuclei. The Golgi apparatus produces vesicles, which move along the microtubules to the region of the disk. As more vesicles arrive and fuse, a cell plate can be seen. The cell plate is simply a newly formed plasma membrane that expands outward until it reaches the old plasma membrane and fuses with this membrane.

Explain the results of the experiment

For this experiment we ran five different treatments to try to increase the root development on the shoots. The first treatment was the control and the cuttings were stuck into the sand. In the second treatment, we tried to throw an external queue at the plant cells to make them think it is spring by soaking them in water. After 24 hours in the water we put them in a bucket just like the control. For the next treatment we tried to bypass the process of internal and external queues and just put hormones directly on the shoots; and we can do that because they sell rooting hormone at the nursery and we just sprinkled the hormone right on the shoots to see if it would trigger them to divide and become roots. After putting the rooting hormone directly on the shoots, we combined it with the water and see if the combination of soaking plus the rooting hormone has a better effect. Our last treatment went along the theme of not spending a bunch of money on the restoration. The last treatment was putting dogwood cuttings in with willow cuttings, along with water, and let them sit for 24 hours before planting them. Overall, what we did was put a bunch of tree stem cuttings in different conditions to try to get their cells to divide rapidly and grow new roots. After a few weeks it was time to check them and see if we had any success. On the surface it looked like everything was going really well because we could see leaves growing from the cuttings. That means cell division did occur to make the leaves, but we wanted to make sure it occurred below ground as well to make the roots.We carefully dump out each bucket into a container to get shoots separated from the dirt and then rinsed them in a bucket of water so we get a clear view of any roots that are growing from the stems. We definitely saw some roots on the part of the cuttings that were in the soil but we wanted to go through and look at them a little bit closer to come up with a ranking for which ones work best and which ones didn't work so well. Our results showed that the last place treatment is going to go to the cuttings that were soaked in water for 24 hours. They had the lowest amount of root growth on them. The definite winners were the willow cuttings and rooting powders. Those worked because they sent some chemical signals to the cells of these shoots to tell the cells to divide and tell them to differentiate into root cells. And they grew really fast. If we looked at these under the microscope, we would see lots of cells in the phases of mitosis like prophase, metaphase, anaphase and telophase because they are rapidly dividing down here in these new roots that are growing. Even though it was the most expensive option, it was definitely the most effective so we are going to use the rooting powder and water treatment on the rest of the cuttings when it is time to do the habitat restoration this spring.

which is the longest phase of the cell cycle for most cells

G1

arrange the phases of the cell cycle in order

G1 S G2 M C

The cell doubles its organelles and accumulates the materials needed for DNA synthesis.

G1 stage

Meiosis Is Reduction Division

The central purpose of meiosis is to reduce the chromosome number from 2n to n. Meiosis requires two nuclear divisions and produces four haploid daughter cells, each having one of each kind of chromosome. The process begins by replicating the chromosomes, then splitting the matched homologous pairs to go from 2n to n chromosomes during the first division. The second division reduces the amount of DNA in n chromosomes to an amount appropriate for each gamete. Once the DNA has been replicated and chromosomes become a pair, they may exchange genes, creating a genetic mixture different from the parent. The first nuclear division separates each homologous pair, reducing the chromosome number from 2n to n. Even though each daughter cell now has n chromosomes, each chromosome still has a sister chromatid, making a second nuclear division necessary. The end result of meiosis is four gametes with n chromosomes. Figure 10.2 presents an overview of meiosis, indicating the two nuclear divisions, meiosis I and meiosis II. Prior to meiosis I, DNA replication has occurred; therefore, each chromosome has two sister chromatids. During meiosis I, something new happens that does not occur in mitosis. The homologous chromosomes come together and line up side by side, forming a synaptonemal complex. This process is called synapsis (Gk. synaptos, "united, joined together") and results in a bivalent (L. bis, "two"; valens, "strength")—that is, two homologous chromosomes that stay in close association during the first two phases of meiosis I. Sometimes the term tetrad (Gk. tetra, "four") is used instead of bivalent, because, as you can see, a bivalent contains four chromatids. Chromosomes may recombine or exchange genetic information during this association (see Section 10.2).

Division of the Centrosome

The centrosome (Gk. centrum, "center"; soma, "body"), the main microtubule-organizing center of the cell, also divides before mitosis begins. Each centrosome in an animal cell contains a pair of barrel-shaped organelles called centrioles. Centrioles are not found in plant cells. The centrosomes organize the mitotic spindle, which contains many fibers, each of which is composed of a bundle of microtubules. Microtubules are hollow cylinders made up of the protein tubulin. They assemble when tubulin subunits join, and when they disassemble, tubulin subunits become free once more. The microtubules of the cytoskeleton disassemble when spindle fibers begin forming. Most likely, this provides tubulin for the formation of the spindle fibers, or it may allow the cell to change shape as needed for cell division.

Characteristics of Cancer Cells

The development of cancer is gradual. A mutation in a cell may cause it to become precancerous, but many other regulatory processes within the body prevent it from becoming cancerous. In fact, it may be decades before a cell possesses most or all of the characteristics of a cancer cell (Table 9.2 and Fig. 9.8). Although cancers vary greatly, cells that possess the following characteristics are generally recognized as cancerous: Cancer cells lack differentiation. Cancer cells are not specialized and do not contribute to the functioning of a tissue. Although cancer cells may still possess many of the characteristics of surrounding normal cells, they usually look distinctly abnormal. Normal cells can enter the cell cycle about 50 times before they are incapable of dividing again. Cancer cells can enter the cell cycle an indefinite number of times and, in this way, seem immortal. Cancer cells have abnormal nuclei. The nuclei of cancer cells are enlarged and may contain an abnormal number of chromosomes. Extra copies of one or more chromosomes may be present. Often, there are also duplicated portions of some chromosomes present, which causes gene amplification, or extra copies of specific genes. Some chromosomes may also possess deleted portions. Cancer cells do not undergo apoptosis. Ordinarily, cells with damaged DNA undergo apoptosis, or programmed cell death. The immune system can also recognize abnormal cells and trigger apoptosis, which normally prevents tumors from developing. Cancer cells fail to undergo apoptosis even though they are abnormal cells. Cancer cells form tumors. Normal cells anchor themselves to a substratum and/or adhere to their neighbors. They exhibit contact inhibition—in other words, when they come in contact with a neighbor, they stop dividing. Cancer cells have lost all restraint and do not exhibit contact inhibition. The abnormal cancer cells pile on top of one another and grow in multiple layers, forming a tumor. During carcinogenesis, the most aggressive cell becomes the dominant cell of the tumor. Cancer cells undergo metastasis and angiogenesis. Additional mutations may cause a benign tumor, which is usually contained within a capsule and cannot invade adjacent tissue, to become malignant, and spread throughout the body, forming new tumors distant from the primary tumor. These cells now produce enzymes that they normally do not express, allowing tumor cells to invade underlying tissues. Then, they travel through the blood and lymph, to start tumors elsewhere in the body. This process is known as metastasis. Tumors that are actively growing soon encounter another obstacle—the blood vessels supplying nutrients to the tumor cells become insufficient to support the rapid growth of the tumor. In order to grow further, the cells of the tumor must receive additional nutrition. Thus, the formation of new blood vessels is required to bring nutrients and oxygen to support further growth. Additional mutations occurring in tumor cells allow them to direct the growth of new blood vessels into the tumor in a process called angiogenesis. Some modes of cancer treatment are aimed at preventing angiogenesis from occurring.

Trisomy 21

The most common autosomal trisomy seen among humans is trisomy 21, also called Down syndrome. This syndrome is easily recognized by these characteristics: short stature; an eyelid fold; a flat face; stubby fingers; a wide gap between the first and second toes; a large, fissured tongue; a round head; a distinctive palm crease; heart problems; and some degree of intellectual disability, which can sometimes be severe. Individuals with Down syndrome also have a greatly increased risk of developing leukemia and tend to age rapidly, resulting in a shortened life expectancy. In addition, these individuals have an increased chance of developing Alzheimer disease later in life. The chances of a woman having a child with Down syndrome increase rapidly with age. In women ages 20 to 30, the incidence of trisomy 21 is 1 in 1,400 births; in women 30 to 35, the incidence is about 1 in 750 births. It is thought that the longer the oocytes are stored in the female, the greater the chances of nondisjunction occurring. However, even though an older woman is more likely to have a Down syndrome child, most babies with Down syndrome are born to women younger than age 40, because this is the age group having the most babies. Furthermore, research indicates that in 23% of the cases studied, the sperm contributed the extra chromosome. A karyotype, a visual display of the chromosomes arranged by size, shape, and banding pattern, may be performed to identify babies with Down syndrome and other aneuploid conditions (Fig. 10.12) Why are newborns with an abnormal sex chromosome number more likely to survive than those with an abnormal autosome number? Because females have two X chromosomes and males have only one, we might expect females to produce twice the amount of each gene from this chromosome, but both males and females produce roughly the same amount. In reality, both males and females only have one functioning X chromosome. In females, and in males with extra X chromosomes, any additional X chromosomes become an inactive mass called a Barr body, named after Murray Barr, the person who discovered it. This inactivation provides a natural method for gene dosage compensation of the sex chromosomes (see Section 13.2) and explains why extra sex chromosomes are more easily tolerated than extra autosomes.

Oogenesis

The ovaries contain stem cells, called oogonia, that produce many primary oocytes with 46 chromosomes during fetal development. They even begin oogenesis, but only a few continue Page 177when a female has become sexually mature. The result of meiosis I is two haploid cells with 23 chromosomes each (Fig. 10.10). One of these cells, termed the secondary oocyte, receives almost all the cytoplasm. The other is a polar body that may either disintegrate or divide again.

What was the purpose of this experiment?

The purpose of this experiment stemmed from the fact that a large piece of property used to be a cow pasture so there really isn't a lot left on the land as far as cover for wild life goes. In the spring, there is a goal of completing habitat restoration by planting a bunch of trees. As we all know, buying trees, especially as many as we needed, would be extremely expensive. The goal was to increase what is already on the property in order to provide more cover for the wildlife. On the land, there is already a shrub called red ojer dogwood and it is great for wildlife. The birds like it because they use it for nesting as well as for the berries it produces, and deer love it as well. The red twigs on the shrub are really nutritious and in the winter they provide good burrows. The dogwood seedlings are sold at the nursery but they run about ten to twenty dollars a piece, but for the amount we need to cover the property it would cost way too much. So, this experiment was to figure out a way to multiply what we already have. When researching habitat restoration, there was information that certain species like dogwoods and willows can start an entire new plant just from the stem cutting from an already growing plant. Overall, the purpose of this experiment was to take some of the stem cuttings from the red ojer dogwood shrub inside and do a couple different experiments in order to find the most effective way to get the stems to start new roots and grow into a new plant in order to complete habitat restoration on a large piece of property come springtime.

The Cycle of Life

The term life cycle refers to all the reproductive events that occur from one generation to the next similar generation. In animals, including humans, the individual is always diploid, and meiosis produces the gametes, the only haploid phase of the life cycle (Fig. 10.8). In contrast, plants have a haploid phase that alternates with a diploid phase. The haploid generation, known as the gametophyte, may be larger or smaller than the diploid generation, called the sporophyte. Animals are diploid, and meiosis occurs during the production of gametes, called gametogenesis. In males, meiosis is a part of spermatogenesis (Gk. sperma, "seed"), which occurs in the testes and produces sperm. In females, meiosis is a part of oogenesis (Gk. oon, "egg"), which occurs in the ovaries and produces eggs. A sperm and an egg join at fertilization, restoring the diploid chromosome number. The resulting zygote undergoes mitosis during development of the fetus. After birth, mitosis is involved in the continued growth of the child and the repair of tissues at any time.

Tissue

Tissue is a cellular organizational level between cells and a complete organ. Tissues are groups of cells that have a similar structure and act together to perform a specific function. Organs are formed by multiple tissues grouping together.

Turner Syndrome

Turner females are short, with a broad chest and widely spaced nipples. These individuals also have a low posterior hairline and neck webbing. The ovaries, oviducts, and uterus are very small and underdeveloped. Turner females do not undergo puberty or menstruate, and their breasts do not develop. However, some have given birth following in vitro fertilization using donor eggs. They usually are of normal intelligence and can lead fairly normal lives if they receive hormone supplements.

Changes in Chromosome Number and Structure

We have seen that crossing-over creates variation within a population and is essential for the normal separation of chromosomes during meiosis. Furthermore, the proper separation of homologous chromosomes during meiosis I and the separation of sister chromatids during meiosis II are essential for the maintenance of normal chromosome numbers in living organisms. Although meiosis almost always proceeds normally, a failure of chromosomes to separate, or nondisjunction, may occur, resulting in a gain or loss of chromosomes. Errors in crossing-over may result in extra or missing parts of chromosomes.

Genetic Variation

We have seen that meiosis provides a way to keep the chromosome number constant generation after generation. Without meiosis, the chromosome number of the next generation would continually increase. The events of meiosis also help ensure that genetic variation occurs with each generation.

Deletion Syndrome

Williams syndrome occurs when chromosome 7 loses a tiny end piece (see Fig. 10.15). Children who have this syndrome have turned-up noses, wide mouths, 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 is seen when chromosome 5 is missing an end piece. The affected individual has a small head, an intellectual disability, 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.