biology quiz nine

Bring together the molecular mechanisms that help regulate the cell cycle by first labeling the figure below, then explain at each number what is occurring in the cell at that point.

1. Synthesis of cyclin begins in late @ phase and continues through G2. Because cyclin is protected from degradation during this stage, it accumulates. 2. Cyclin combines with Cdk, producing MPF. When enough MPF molecules accumulate, the cell passes the G2 checkpoint and begins mitosis. 3. MPF promotes mitosis by phosphorylating various proteins. MPF's activity peaks during metaphase. 4. During anaphase, the cyclin component of MPF is degraded, terminating the M phase. The cell enters the G1 phase. 5. During G1, the degradation of cyclin continues, and the Cdk component of MPF is recycled.

How many chromosomes are present in a human gamete?

23

How many chromosomes are present at the end of anaphase I?

3

How many chromosomes are present at the end of telophase I?

3

A chicken has 78 chromosomes in its somatic cells. How many chromosomes did the chicken inherit from each parent? How many chromosomes are in each of the chicken's gametes? How many chromosomes will be in each somatic cell of the chicken's offspring?

39; 39; 78

​How many DNA molecules are in each of your somatic cells?

46

How many chromosomes does the cell have? How many homologous pairs? How many chromatids? Is this cellhaploid (n) or diploid (2n)?

6; 3; 12; diploid (2n)

chromosome

A cellular structure carrying genetic material, found in the nucleus of eukaryotic cells. Each chromosome consists of one very long DNA molecule and associated proteins. (A bacterial chromosome usually consists of a single circular DNA molecule and associated proteins. It is found in the nucleoid region, which is not membrane bounded.)

Describe the chromosomes of the daughter cells

A chromosome is one of the two chromatids from a homologous pair with some genetic exchange with its homolog during crossing over in prophase 1.

gene

A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses).

What is a kinetochore?

A kinetochore is a structure of proteins that have assembled on specific sections of DNA at the centromere of each sister chromatid. The kinetochore links each sister chromatid to the mitotic spindle.

What are homologs (homologous chromosomes)?

A pair of chromosomes of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci

locus

A specific place along the length of a chromosome where a given gene is located

sexual reproduction

A type of reproduction in which two parents give rise to offspring that have unique combinations of genes inherited from both parents via the gametes.

somatic cells

All cells of the body except the gametes and their precursors.

During which parts of the cell cycle does the cell grow?

All three phases of interphase

What is the function of a protein kinase?

An enzyme that transfers phosphate groups from ATP to a protein, which may activate or inactivate the protein.

Which type of reproduction will result in genetically identical offspring?

Asexual reproduction

What are the three key roles of cell division

Asexual reproduction gives rise to a new organism that is genetically identical to the parent. Growth and development enables multicellular eukaryotes to develop from a single cell-the fertilized egg-to a two-celled embryo, the first stage in development. Tissue renewal and repair replaces cells that die from accidents or normal wear and tear in a multicellular organism.

independent assortment of chromosomes

At metaphase I, the homologous pairs, each consisting of one maternal and one paternal chromosome, are situated at the metaphase plate. Each pair may orient with either its maternal or paternal homolog closer to a given pole—its orientation is as random as the flip of a coin. Thus, there is a 50% chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a 50% chance that it will get the paternal chromosome. Because each pair of homologous chromosomes is positioned independently of the other pairs at metaphase I, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair. This is called independent assortment. Each daughter cell represents one outcome of all possible combinations of maternal and paternal chromosomes.

what is the function of the centrosomes?

Centrosomes are regions in animal cells that organize the microtubules of the spindle. Each centrosome contains two centrioles.

crossing over

Crossing over begins very early in prophase I as homologous chromosomes pair loosely along their lengths. Each gene on one homolog is aligned precisely with the corresponding gene on the other homolog. In a single crossover event, the DNA of two nonsister chromatids—one maternal and one paternal chromatid of a homologous pair—is broken by specific proteins at precisely corresponding points, and the two segments beyond the crossover point are each joined to the other chromatid. Thus, a paternal chromatid is joined to a piece of maternal chromatid beyond the crossover point, and vice versa. In this way, crossing over produces chromosomes with new combinations of maternal and paternal alleles.

The signature event of crossing over occurs during prophase I. Using Figure13.9, label then thoroughly explain what occurs in the four steps shown by the numbers in the diagram.

Crossing over, a genetic rearrangement between nonsister chromatids involving the exchange of corresponding segments of DNA molecules, has the following steps: 1.​The homologous chromosomes pair and associate along their length. The DNA molecules are broken at precisely corresponding points in the nonsister chromatids. The chromatin condenses. 2.​The synaptonemal complex is formed attaching one homolog to the other. The chromatin continues to condense. 3.​The synaptomemal complex is fully formed, and the homologs are in synapsis. The DNA breaks are closed up when each broken end is joined to the corresponding segment of the nonsister chromatid, producing a crossover. 4.​After the synaptonemal complex disassembles, the homologs move slightly apart but remain attached because of sister chromatid cohesion. The points of attachment show up as chiasmata and indicate where crossovers have occurred

Kinases drive the cell cycle, but they must be activated by attachment of what molecules?

Cyclins

Describe cytokinesis in a plant cell. Make a labeled sketch that shows the cell plate.

Cytokinesis in plant cells, which have cell walls, is markedly different. There is no cleavage furrow. Instead, during telophase, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a cell plate. Cell wall materials carried in the vesicles collect in the cell plate as it grows. The cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell. Two daughter cells result, each with its own plasma membrane. Meanwhile, a new cell wall arising from the contents of the cell plate has formed between the daughter cells.

The figure below shows metaphase I of meiosis. How is the arrangement of chromosomes here different from the metaphase of mitosis?

During mitosis, individual chromosomes line up at the metaphase plate. During meiosis, pairs of homologous chromosomes line up at the metaphase plate.

Explain the difference between kinetochore and nonkinetochore microtubules. What is the function of each?

During prometaphase, some of the spindle microtubules attach to the kinetochores; these are called kinetochore microtubules. Their function is to jerk the chromosomes back and forth. Meanwhile, microtubules that do not attach to kinetochores, nonkinetochore microtubules, interact with those from the opposite pole of the spindle, lengthening the cell.

It is easy to have the misconception that the chromosomes inherited from our parents are exclusively maternal or paternal. The chromosomes we inherit are actually recombinant chromosomes. Explain what this means and clarify the misconception

During prophase 1 of meiosis homologous chromosomes synapse and cross over, therefore the chromosomes in the gametes produced are a recombination of the maternal and paternal chromosomes. Those chromosomes that we transmit to our offspring are recombined chromosomes from our mother and father (their grandparents).

Summarize what happens at each checkpoint

G1 If a cell receives the go-ahead signal at this checkpoint, the cell cycle will continue. It is regulated by the cyclin-Cdk complexes. G2 When sufficient MPF accumulates, the G2 checkpoint is passed, and mitosis is promoted. M The kinetochores must all be attached to spindle fibers during metaphase. This will activate an enzyme (separase), which allows the sister chromatids to separate, and anaphase will proceed.

Label each of the parts of the cell cycle listed below and give a brief explanation of what happens in each phase

G1:​"First gap," the first part of interphase. Metabolic activity and growth occurs in this phase. S:​"Synthesis," the second phase of interphase. DNA synthesis occurs in preparation for mitosis. Metabolic activity and growth also occur. G2:​"Second gap," the last part of interphase. Metabolic activity, growth, and preparation for cell division occur. M:​In the M phase, mitosis distributes the daughter chromosomes to daughter nuclei, and cytokinesis divides the cytoplasm, producing two daughter cells

What are growth factors? How does platelet-derived growth factor (PDGF) stimulate fibroblast division?

Growth factor is a protein released by certain cells that stimulates other cells to divide. Fibroblasts have PDGF receptors on their plasma membranes. The binding of PDGF molecules to these receptors triggers a signal transduction pathway that allows the cells to pass the G1 checkpoint and divide.

gametes

Haploid reproductive cells, such as egg or sperm cells. Gametes unite during sexual reproduction to produce a diploid zygote.

In meiosis, what follows the duplication of chromosomes?

Homologous chromosomes are separated.

What happens if all the chromosome kinetochores are not attached to spindle fibers? When this occurs, which checkpoint is not passed?

If all the chromosome kinetochores are not attached to spindle fibers, the sister chromatids remain together, delaying anaphase. When this occurs, the M phase checkpoint is not passed.

​centromere

In a duplicated chromosome, the region on each sister chromatid where they are most closely attached to each other by proteins that bind to specific DNA sequences; this close attachment causes a constriction in the condensed chromosome

Which part of the cell cycle is longest in duration

Interphase

What does MPF trigger? What are some specific activities that it triggers?

MPF triggers the cell's passage past the G2 checkpoint into M phase. MPF promotes mitosis by phosphorylating various proteins of the nuclear lamina, which promote fragmentation of the nuclear envelope during prometaphase of mitosis. MPF is also involved in chromosome condensation and spindle formation during prophase.

​By what process are gametes produced?

Meiosis

During which division is the chromosome number reduced?

Meiosis I

​What is the purpose of meiosis?

Meiosis is a modified type of cell division in sexually reproducing organisms consisting of two rounds of cell division but only one round of DNA replication. It results in cells with half the number of chromosome sets as the original cell, producing gametes, and introduces genetic variability.

Many of the events of mitosis depend on the mitotic spindle. In animal cells, the assembly of spindle microtubules starts at the centrosome. What is another name for the centrosome?

Microtubule-organizing center

What is the chromosome number of the daughter cells of meiosis?

N

​chromatid

One of two identical joined copies of the original chromosome.

Mitosis vs. Meiosis

Role in the animal body - Mitosis: Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Meiosis: Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes Number of DNA replications - Mitosis: 1 Meiosis: 1 Number of divisions - Mitosis: 1 Meiosis: 2 Number of daughter cells - Mitosis: 2 Meiosis: 4 Chromosome number of daughter cells - Mitosis: 2n Meiosis: n Are the daughter cells identical to or different from the parent cell? Mitosis: identical Meiosis: different

Distinguish between sex chromosomes and autosomes. How many of each are found in human cells?

Sex chromosome: A chromosome responsible for determining the sex of an individual: 2 (1 pair) Autosome: A chromosome that is not directly involved in determining sex; not a sex chromosome: 44 (22 pairs)

During meiosis I, homologous chromosomes separate. What separates during meiosis II?

Sister chromatids separate during meiosis II.

chiasmata

The X-shaped, microscopically visible region where crossing over has occurred earlier in prophase I between homologous nonsister chromatids. Chiasmata become visible after synapsis ends, with the two homologs remaining associated due to sister chromatid cohesion.

et's take a moment to consider the question "why sex?" Any individual passes on more of its genes through asexual reproduction, so there must be some advantage to sexual reproduction, as it is almost universal among animals. Under what conditions is sexual reproduction most advantageous, and why?

The ability of sexual reproduction to generate genetic diversity provides an advantage to a population that lives in a changing environment. As the environment changes, the population may survive if, in each generation, at least some of its members can cope effectively with the new conditions.

Cytokinesis

The cell divides into two daughter cells, genetically identical to each other and to the parent cell.

Interphase

The cell grows; in preparation for cell division, the chromosomes are duplicated, with the genetical material (DNA) copied precisely.

Mitosis

The chromosome copies are separated from each other and moved to opposite ends of the cell

​chromatin

The complex of DNA and proteins that makes up eukaryotic chromosomes. When the cell is not dividing, chromatin exists in its dispersed form, as a mass of very long, thin fibers that are not visible with a light microscope

genomes

The entire genetic endowment that consists of all of the DNA contained within the organism's chromosomes.

asexual reproduction

The generation of offspring from a single parent that occurs without the fusion of gametes (by budding, division of a single cell, or division of the entire organism into two or more parts). In most cases, the offspring are genetically identical to the parent.

What occurs in meiosis? Where does it occur in humans?

The human body produces gametes—eggs or sperm—by a variation of cell division called meiosis, which yields nonidentical daughter cells that have only one set of chromosomes (23), half as many chromosomes as the parent cell. ​​Meiosis occurs in special cells in the male testes and in the female ovaries

synapsis

The pairing and physical connection of duplicated homologous chromosomes during prophase I of meiosis.

​random fertilization

The random nature of fertilization adds to the genetic variation arising from meiosis. In humans, each male and female gamete represents one of about 8.4 million possible chromosome combinations due to independent assortment. The fusion of a male gamete with a female gamete during fertilization will produce a zygote with any of about 70 trillion diploid combinations.

​crossing over

The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis.

How many chromosomes are in human cells? What is a chromosome?

There are 46 chromosomes in human somatic cells. A chromosome is a cellular structure carrying genetic material, found in the nucleus of eukaryotic cells. Each chromosome consists of one very long DNA molecule and associated proteins.

What happens to the chromosome number in meiosis?

They are reduced by half

​What are the components of the mitotic spindle? What is the source of these components?

This structure consists of fibers made of microtubules and associated proteins. While the mitotic spindle assembles, the other microtubules of the cytoskeleton partially disassemble, providing the material used to construct the spindle. The spindle microtubules elongate (polymerize) by incorporating more subunits of the protein tubulin and shorten (depolymerize) by losing subunits. The source of these components is the cytoplasm during prophase.

What is the purpose of meiosis?

To reduce the number of sets of chromosomes from two to one in gametes

The muscle cells of a dog have 78 chromosomes. Fill in the correct chromosome number for the following cells in a dog

bone cell: 78 sperm: 39 haploid cell: 39 somatic cell: 78 zygote: 78

female gamete

eggs

How many daughter cells are formed at the conclusion of meiosis?

four

​How many daughter cells are formed?

four

Distinguish between a benign tumor and a malignant tumor.

genetic and cellular changes such that the cells are not capable of surviving at a new site and generally remain at the site of the tumor's origin. ​​A malignant tumor is a cancerous tumor containing cells that have significant genetic and cellular changes and are capable of invading and surviving in new sites. Malignant tumors can impair the functions of one or more organs.

At the end of the first meiotic division, are the daughter cells haploid or diploid?

haploid

Cells that have only one of each homologous pair are said to be haploid, a condition that is represented by n. Cells that have two of each homologous pair are said to be diploid or 2n. For each of the following, is the cell haploid or diploid?

liver cell: diploid gamete: haploid egg cell: haploid zygote: diploid skin cell: diploid sperm: haploid somatic cell: diploid stem cell: diploid

How many times do the chromosomes duplicate?

none

male gamete

sperm

Describe the three events unique to meiosis that occur during meiosis I (1)

synapsis and crossing over: during prophase 1, duplicated homologous chromosomes pair up and cross over alignment of chromosome pairs at the metaphase plate: pairs of homologous chromosomes position at the metaphase plate rather than individual chromosomes separation of homologs: Chromosomes of each homologous pair move toward opposite poles, but the sister chromatids remain attached

How many times does the cell divide in meiosis?

twice

​What is a cell cycle checkpoint?

​A control point in the cell cycle where stop and go-ahead signals can regulate the cycle

explain how evidence was gathered to justify the claim that microtubules depolymerize from the kinetochore end during anaphase.

​A fluorescent yellow dye was used to label microtubules of a pig kidney cell in early anaphase. A laser was then used to mark a region (remove fluorescence) of the microtubules between one spindle pole and the chromosomes. As the chromosomes moved poleward, the microtubule segments on the kinetochore side of the mark shortened, whereas those on the spindle pole side stayed the same length

What is a gamete? Name the two types of gametes.

​A gamete is a reproductive cell, either sperm or egg

​What is the meaning of genome?

​A genome is a cell's DNA, its genetic information.

What are alleles? Give an example.

​Alleles are any of the alternative versions of a gene that may produce distinguishable phenotypic effects. A possible example is the allele for freckles.

What is the Go phase? Describe this phase. What cell types remain in this phase throughout their life spans?

​G0 phase is a nondividing state occupied by cells that have left the cell cycle, sometimes reversibly, such as liver cells. Most cells of the human body are actually in this phase

Cancer cells also show loss of cell cycle controls and may divide without being checked. The story of HeLa cells is worth noting. What is their source? How old are they? Note that, unlike normal cells, HeLa cells are immortal!

​HeLa cells original source was a tumor removed from a woman named Henrietta Lacks. This cell line has been reproducing in culture since 1951

​Describe cytokinesis in an animal cell. Make a labeled sketch that shows the cleavage furrow.

​In animal cells, cytokinesis occurs by a process known as cleavage. The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate. On the cytoplasmic side of the furrow is a contractile ring of actin microfilaments associated with molecules of the protein myosin. The actin microfilaments interact with the myosin molecules, causing the ring to contract. The contraction of the dividing cell's ring of microfilaments is like the pulling of a drawstring. The cleavage furrow deepens until the parent cell is pinched in two, producing two completely separated cells, each with its own nucleus and share of cytosol, organelles, and other subcellular structures.

Explain what is meant by homologous chromosomes

​In diploid cells, the two chromosomes of a pair that have the same length, centromere position, and staining pattern visible in a karyotype are homologous chromosomes.

​There are two divisions in meiosis. What will separate in the first division in meiosis I?

​In meiosis I, homologous chromosomes separate

What is mitosis? How is it different from cytokinesis?

​Mitosis, the division of the genetic material in the nucleus, is usually followed immediately by cytokinesis, the division of the cytoplasm.

What controls the cell cycle?

​Molecules present in the cytoplasm during the S or M phase control the progression to those phases

Why must crossing over occur with a nonsister chromatid?

​Sister chromatids are identical, so crossing over would not yield a different combination of genes. The association of the synaptonemal complex and the processes that follow are formed with nonsister chromatids.

How does a somatic cell compare to a gamete in terms of chromosome number?

​Somatic cells contain a double set of chromosomes; they are 2n or diploid. Gametes contain a single set of chromosomes; they are (n) or haploid. For humans, each somatic cell has 46 chromosomes and a gamete cell (an egg or sperm) has 23 chromosomes.

The activity of cyclin-dependent kinases (CDks) rises and falls. Why?

​The activity of CDKs is directly associated with the cytoplasmic cyclin concentration, which begins to accumulate in late S phase and continues to accumulate until anaphase of mitosis, where it is degraded.

​What is considered to be the cell cycle?

​The cell cycle is the life of a cell from the time it is first formed from a dividing parent cell until its own division into two daughter cells.

Describe what happens to the centrosome during prophase.

​The centrosomes move away from each other, propelled partly by the lengthening microtubules between them

What is chromatin?

​The entire complex of DNA and proteins that is the building material of chromosomes. Chromatin will vary in its degree of condensation during the process of cell division.

​Where are the gametes of an animal produced? Be specific as to male and female gametes.

​The only cells of the human body not produced by mitosis are the gametes, which develop from specialized cells called germ cells in the gonads—ovaries in females and testes in males.

How many chromosomes are in a human somatic cell? Name two types of somatic cells in your body.

​There are 46 chromosomes in a human somatic cell. Possible examples of somatic cells include any cells in the body (lung cells, skin cells), except for reproductive cells

What is transformation? What is metastasis?

​Transformation is the conversion of a normal animal cell to a cancerous cell. ​​Metastasis is the spread of cancer cells to locations distant from their original site.

What is a karyotype? How is it prepared?

​When images of the chromosomes are arranged in pairs, starting with the longest chromosomes, the resulting ordered display is called a karyotype. Karyotypes are prepared from isolated somatic cells, which are treated with a drug to stimulate mitosis and then grown in culture for several days. Cells arrested in metaphase, when chromosomes are most highly condensed, are stained and then viewed with a microscope equipped with a digital camera. A photograph of the chromosomes is displayed on a computer monitor, and the images of the chromosomes are arranged into pairs according to their appearance.

Label the following figure and summarize what occurs at the DNA level in each stage. The top figure shows one chromosome. The middle figure shows a duplicated chromosome with two sister chromatids. It is still considered one chromosome. When the sister chromatids have separated in the bottom figure, they are now considered individual chromosomes.

​​One of the multiple chromosomes in a eukaryotic cell is represented here, not yet duplicated. Normally it would be a long, thin chromatin fiber containing one DNA molecule and associated proteins; here its condensed form is shown for illustration purposes only. ​​Once duplicated, a chromosome consists of two sister chromatids connected along their entire lengths by sister chromatid cohesion. Each chromatid contains a copy of the DNA molecule. ​​Molecular and mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells.

List two specific cancer treatments and explain how each treatment works.

​​Radiation, chemotherapy ​​A tumor that appears to be localized may be treated with high-energy radiation, which damages DNA in cancer cells much more than it does in normal cells, apparently because the majority of cancer cells have lost the ability to repair such damage. To treat known or suspected metastatic tumors, chemotherapy is used, in which drugs that are toxic to actively dividing cells are administered through the circulatory system. Chemotherapeutic drugs interfere with specific steps in the cell cycle.