Genetics Chapter 2

Name three essential structural elements of a functional eukaryotic chromosome and describe their functions.

1. Centromere is a constricted region drawing in chromosomes into the newly formed nuclei 2. A pair of telomeres are the natural ends of a whole chromosome protecting and stabilizing it. 3. Origin of replication is the site where DNA replication begins.

List the stages of interphase and the major events that take place in each stage.

1. G1 cells grow and synthesis proteins necessary for cell division 2. S chromosomes replicate 3. G2 several different biochemical events take place that are necessary for division

Outline the processes involved in oogenesis including mitosis, cell growth, the two divisions of meiosis, the unequal division of cytoplasm, and the degeneration of polar bodies.

1. In the ovaries of a female fetus, cells called oogonia (2n) divide by mitosis to make more oogonia cells. 2. Oogonia grow into larger cells called primary oocytes (2n). 3. Primary oocytes start meiosis but stop at prophase I. Primary oocytes and a single layer of follicle cells form a primary follicle. 4. When a baby girl is born, the ovaries contain 400,000 primary follicles. 5. Every menstrual cycle, a few primary follicles start to develop -- During this time, the Primary oocyte completes meiosis I, forming two haploid (n) daughter nuclei - cytoplasm of the primary oocyte develops unequally, forming a secondary oocyte (n) and a small polar body (n). 6. Then, the Secondary oocyte starts second division of meiosis but stops at prophase II. The follicle cells, meanwhile, are reproducing quickly and follicle fluid is forming. 7. When the mature follicle bursts at the time of ovulation, the released egg is still a secondary oocyte (n). 8. After fertilization, the secondary oocyte completes the second division of meiosis to form an ovum with the sperm nucleus already inside.

List three fundamental events that must take place in cell reproduction.

1. Its genetic information must be copied. 2. The copies of genetic information must be separated from each other 3. The cells must divide.

Outline the processes involved in spermatogenesis including mitosis, cell growth, the two divisions of meiosis, and cell differentiation.

1. The outer layer of cells, called spermatogonia (2n) divides by mitosis, producing more spermatogonia (also 2n). 2. Spermatogonia grow into larger cells called primary spermatocytes (2n). 3. Primary spermatocytes (2n) carry out the first stage of meiosis, producing 2 secondary spermatocytes (n). 4. Each secondary spermatocyte carries out the second division of meiosis, producing four spermatids (n). 5. Spermatids become associated with nurse cells called sertoli cells, which help the spermatids develop into spermatozoa (n). 6. Mature sperm (the spermatozoa) detach from sertoli cells and eventually are carried out of the testes by fluid in the center of the seminiferous tubule.

A cell in prophase II of meiosis contains 12 chromosomes. How many chromosomes would be present in a cell from the same organism if it were in prophase I of meiosis?

24

Normal somatic cells of horses have 64 chromosomes (2n = 64). How many chromosomes and DNA molecules will be present in the following types of horse cells? b. First polar body

32 chromo 64 DNA molecules (The first polar body is the product of meiosis I so it will be haploid, but the sister chromatids have not separated so each chromosome will consist of two sister chromatids.)

Normal somatic cells of horses have 64 chromosomes (2n = 64). How many chromosomes and DNA molecules will be present in the following types of horse cells? d. Secondary spermatocyte

32 chromo 64 DNA molecules (The secondary spermatocyte is a product of meiosis I and has yet to enter meiosis II. So, the secondary spermatocyte will be haploid because the homologous pairs were separated in meiosis I, but each chromosome is still composed of two sister chromatids.)

Normal somatic cells of horses have 64 chromosomes (2n = 64). How many chromosomes and DNA molecules will be present in the following types of horse cells? c. Primary oocyte

64 chromo 128 DNA molecules (The primary oocyte has stopped in prophase I of meiosis. So, the homologs have not yet separated and each chromosome consists of two sister chromatids.)

Normal somatic cells of horses have 64 chromosomes (2n = 64). How many chromosomes and DNA molecules will be present in the following types of horse cells? a. Spermatogonium

64 each (Assuming the spermatogonium is in G1 prior to the production of sister chromatids in S phase, the chromsome number will be the diploid number of chromosomes.)

Difference between a prokaryotic cell and a eukaryotic cell.

A prokaryotic cell possess a simple structure, with no nuclear envelope and usually a single circular chromosome. A eukaryotic cell possesses a more complex structure, with a nucleus and multiple linear chromosomes consisting of DNA complexed with histone proteins.

Where are eukaryotic cells found in homologous pairs?

Chromosomes. Each functional chromosome consists of a centromere, telomeres, and multiple origins of replication. After a chromosome has been copied, the two copies remain attached at the centromere, forming sister chromatids.

List several characteristics that bacteria and archaea have in common and that distinguish them from eukaryotes.

Bacteria and archaea are prokaryotes. They differ from eukaryotes in possessing no nucleus, a genome that usually consists of a single circular chromosome, and a smaller amount of DNA.

Female bees are diploid, and male bees are haploid. The haploid males produce sperm and can successfully mate with diploid females. Fertilized eggs develop into females and unfertilized eggs develop into males. How do you think the process of sperm production in male bees differs from sperm production in other animals?

Because meiosis takes place only in diploid cells, haploid male bees do not undergo meiosis. Male bees produce sperm only through mitosis. Haploid cells that divide mitotically produce haploid cells.

How are the events that take place in spermatogenesis and oogenesis similar? How are they different?

Both spermatogenesis and oogenesis begin similarly in that the diploid primordial cells (spermatogonia and oogonia) can undergo multiple rounds of mitosis to produce more primordial cells, or both types of cells can enter into meiotic division. In spermatogenesis, cytokinesis is equal, resulting in haploid cells of similar sizes. Upon completion of meiosis II, four haploid spermatids have been produced for each spermatogonium that began meiosis. In oogenesis, cytokinesis is unequal. At the completion of meiosis I in oogenesis, a secondary oocyte is produced, which is much larger and contains more cytoplasm than the other haploid cell produced, called the first polar body. At the completion of meiosis II, the secondary oocyte divides, producing the ovum and the second polar body. Again, the division of the cytoplasm in cytokinesis is unequal, with the ovum receiving most of the cytoplasmic material. Usually, the polar bodies disintegrate, leaving the ovum as the only product of meiosis.

Which of the following events takes place in metaphase I? a. Crossing over occurs. b. Chromosomes condense. c. Homologous pairs of chromosomes line up on the metaphase plate. d. Individual chromosomes line up on the metaphase plate.

C.

How is progression through the cell cycle regulated?

Checkpoints that allow or prohibit the cell's progression to the next stage.

What are checkpoints? List some of the important checkpoints in the cell cycle.

Check point ensure proper cell division takes place. 1. G1/S checkpoint will hold the cell at G1 until the cell has all the enzymes necessary for division. 2. G2/M checkpoint makes sure all DNA is present with no damage 3. Spindle assembly checkpoint makes sure spindle fiber tension is correct.

Briefly describe how the chromosomes move toward the spindle poles during anaphase.

Chromosome movement is due to the disassembly of tubulin molecules at both the kinetochore ad the spindle end

What is cohesin and what does it do?

Cohesin holds sister chromatids together. In metaphase of mitosis and in metaphase II of meioisis, the breakdown of cohesin allows sister chromatids to separate. In meiosis I, centromeric cohesin remains intact and keeps sister chromatids together so that homologous chromosomes, but not sister chromatids, separate in anaphase I.

What does cell reproduction require?

Copying of genetic material, separation of the copies, and cell division.

What two processes unique to meiosis are responsible for genetic variation? At what point in meiosis do these processes take place?

Crossing over occurs in prophase 1 when genetic material is exchanged between nonsister chromatids. Random separation occurs in anaphase 1 after a random aliment in metaphase 1.

A secondary spermatocyte has 12 chromosomes. How many chromosomes will be found in the primary spermatocyte that gave rise to it? a. 6 b. 12 c. 18 d. 24

D

Which of the following structures is diploid? a. Microspore b. Egg c. Megaspore d. Microsporocyte

D

A horse has 64 chromosome and a donkey has 62.Using your knowledge of meiosis, evaluate why a cross between a horse and a donkey produces a mule, which usually is sterile.

During Metaphase 1, different chromosomes will not be able to find their homologous pair, so mules cannot normally make sex cells.

How does shugoshin affect sister chromatids in meiosis I and meiosis II?

During anaphase I, shugoshin protects cohesin at the centromeres from the action of separase, so centromeric cohesin remains intact, and the sister chromatids remain together. Subsequently, shugoshin breaks down, so centromeric cohesin is cleaved by separase in anaphase II, and the sister chromatids separate.

What are some genetic differences between prokaryotic and eukaryotic cells?

Eukaryotic cells have a nuclear envelope which surrounds the genetic material to form a nucleus and separates the DNA from the other cellular content. Eukaryotic cells also have DNA in tightly bound forms with histones called chromatid. Genes in prokaryotic cells a on a single circular molecule of DNA.

What is the production of meiosis?

Four haploid cells that are genetically variable. Genetic variation in meiosis is produced by crossing over and by the random distribution of maternal and paternal chromosomes.

What is the correct order of stages in the cell cycle?

G1, S, G2, prophase, metaphase, anaphase

Counting chromosomes of a n=2 (so 4 chromosomes per cell) and DNA molecules. Each cycle.

G1: 4/4 S: 4/8 G2: 4/8 Prophase/metaphase: 4/8 Metaphase: 4/8 Anaphase: 8/8 Telophase/cytokinesis: 4/4

How does reproduction occur in prokaryotic and eukaryotic cells?

In a prokaryotic cell, the single chromosome replicates, the two copies move toward opposite sides of the cell, and the cell divides. In eukaryotic cells, reproduction is more complex than in prokaryotic cells, requiring mitosis, to ensure that a complete set of genetic information is transferred to each new cell, or meiosis, to generate gametes that contain a copy of each chromosome.

How does an animal undergo meiosis with both spermatogonium and oogonium?

In animals, a diploid spermatogonium undergoes meiosis to produce four haploid sperm cells. A diploid oogonium undergoes meiosis to produce one large haploid ovum and one or more smaller polar bodies.

How does anaphase 1 in meiosis differ from anaphase in mitosis?

In meiosis homologous chromosomes separate and in mitosis sister chromatids are separated

Briefly explain why sister chromatids remain together in anaphase I but separate in anaphase II of meiosis.

In meiosis, a similar process to mitosis occurs. Meiosis-specific cohesin complexes (different from cohesion proteins in mitosis) form at the centromeres of the sister chromatids during the S phase. At the beginning of meiosis, cohesin molecules are also found along the entire length of the chromosome arms assisting in the formation of the synaptonemal complex and holding together the two homologs. During anaphase I of meiosis, the cohesin molecules along the arms are cleaved by activated separase allowing the homologs to separate. However, the cohesin complexes at the centromeres of the sister chromatids are protected from the action of separase by the protein shugoshin and are unaffected. The result is that sister chromatids remained attached during anaphase I. At the end of metaphase II, the protection of the cohesin molecules at the centromeres is lost, and the separase proteins can now cleave the cohesin complex, which allows the sister chromatids to separate.

How does a plant undergo meiosis?

In plants, a diploid microsporocyte in the stamen undergoes meiosis to produce four pollen grains, each with two haploid sperm cells. In the ovary, a diploid megasporocyte undergoes meiosis to produce four haploid megaspores. One of these megaspores divides mitotically three times to produce eight haploid nuclei, one of which forms the egg cell and the other fuses with two haploid nuclei to form a triploid endosperm.

A cell has a circular chromosome and no nuclear membrane. Its DNA is complexed to some histone proteins. Does this cell belong to a eubacterium, an arachaea, or a eukaryote?

It belongs to arachaea -Eukaryote and arachaea (ancient bacteria) both have DNA wrapped around histones to form chromatin -Eubacterium (true bacteria) do not contain histones -Eukaryotes have multiple linear chromosomes while arachaea have a single circular chromosome

Stages of cell cycle in eukaryotic cells?

It consists of interphase, in which the cell grows and prepares for division, and the M phase, in which nuclear and cell division takes place. The M phase consists of mitosis, the process of nuclear division, and cytokinesis, the division of the cytoplasm.

What does sexual reproduction do and how does it occur?

Produces genetically variable progeny. It includes meiosis, in which haploid sex cells are produced, and fertilization, the fusion of sex cells. Meiosis includes two cell divisions. In meiosis I, crossing over takes place and homologous chromosomes separate. In meiosis II, sister chromatids separate.

What does mitosis usually result in?

Production of two genetically identical cells.

Compare and contrast the process by which prokaryotic and eukaryotic cells reproduce.

Prokaryotic cells divide by binary fission where after all the genetic material is copied they anchor themselves to the point of origin of replication. After wards a cell wall forms and they split creating two identical cells. Like prokaryotes, Eukaryotic cells must replicate their DNA but it is a bit more complex than the simple cell organisms. DNA is arranged in a more organized fashion within the nucleus and must be copied exactly especially in meiosis.

What would be the result if a chromosome did not have a kinetochore?

The kinetochore is the point at which spindle microtubules attach to the chromosome during cell division. If the kinetochore were missing, spindle microtubules would not attach to the chromosome, the chromosome would not be drawn into a newly formed nucleus, and the resulting daughter cells would be missing a chromosome.

The fruit fly Drosophila melanogaster has four pairs of chromosomes, whereas the house fly Musca domestica has six pairs of chromosomes. All things being equal, in which species would you expect to see more genetic variation among the progeny of a cross? Explain your answer.

The progeny of an organism whose cells contain more homologous pairs of chromosomes should be expected to exhibit more variation. The number of different combinations of chromosomes that are possible in the gametes is 2n, where n is equal to the number of homologous pairs of chromosomes. For the fruit fly with four pairs of chromosomes, the number of possible combinations is 24 = 16. For Musca domestica with six pairs of chromosomes, the number of possible combinations is 26 = 64.

All of the following cells, shown in various stages of mitosis and meiosis, come from the same rare species of plant.

To determine the diploid chromosome number in this plant, the number of centromeres present within a cell that contains homologous pairs of chromosomes must be determined. Remember, each chromosome possesses a single centromere. The location and presence of a centromere are determined by the attachment of the spindle fibers to the chromosome, which occurs at the centromere in the above diagram. Only the cell in stage (a) clearly has homologous pairs of chromosomes. So the diploid chromosome number for cells of this species of plant is six.

A biologist examines a series of cells and counts 160 cells in interphase, 20 cells in prophase, 6 cells in prometaphase, 2 cells in metaphase, 7 cells in anaphase, and 5 cells in telophase. If the complete cell cycle requires 24 hours, what is the average duration of the M phase in these cells? Of metaphase?

To solve this problem, find total number of cells and divide each out. Multiply by 24 to get the time.

Why are the viruses that infect mammalian cells useful for studying the genetics of mammals?

Viruses will evolve depending on the host such as a plant virus will closely resemble a plant cell while an animal virus will closely resemble an animal cell. By studying viruses that mimic its host we can further study the structure of mammal cells.

A primary oocyte divides to give rise to a secondary oocyte and a first polar body. The secondary oocyte then divides to give rise to an ovum and a second polar body. a. Is the genetic information found in the first polar body identical with that found in the secondary oocyte? Explain your answer. b. Is the genetic information found in the second polar body identical with that in the ovum? Explain your answer.

a) No. The first polar body and the secondary oocyte are the result of meiosis I. In meiosis I, homologous chromosomes segregate; thus, both the first polar body and the secondary oocyte will contain only one homolog of each original chromosome pair, and the alleles of some genes in these homologs will differ. Additionally, crossing over in prophase I will have generated new and different arrangements of genetic material for each homolog of the pair.(b) No. The second polar body and the ovum will contain the same members of the homologous pairs of chromosomes that were separated in meiosis I and produced by the separation of sister chromatids in anaphase II. However, the sister chromatids of each pair are no longer identical, because they underwent crossing over in prophase I and thus contain genetic information that is not identical.

29. The amount of DNA per cell of a particular species is measured in cells found at various stages of meiosis, and the following amounts are obtained: Amount of DNA per cell 3.7 pg 7.3 pg 14.6 pg Match the amounts of DNA above with the corresponding stages of the cell cycle (a through f). You may use more than one stage for each amount of DNA. a. G1 b. Prophase I c. G2 d Following telophase II and cytokinesis e. Anaphase I f. Metaphase II

a. 7.3 pg G1 occurs before S phase & the dbling of the DNA. Since 3.7pg was observed and doubling that is ~7.3, we can conclude that 3.7 was found in the haploid cells, so 7.3pg must be in G1 because it is the original amount of DNA b. 14.6 2x the amount of DNA in G1. Homologous chromosomes are located w/in the cell and there are 2 sister chromatids per chromosome c. 14.6 happens after S phase d. 3.7, each daughter cell will contain half amt of DNA of the mother cell in interphase. Both homologous chromosomes and sister chromatids have been separated e. 14.6, 2 sister chromatids per chromosome f. 7.3 (homologous pairs are split up but each chromosome still has 2 sister chromatids

How would each of the following events affect the outcome of mitosis or meiosis? a. Mitotic cohesin fails to form early in mitosis. b. Shugoshin is absent during meiosis. c. Shugoshin does not break down after anaphase I of meiosis. d. Separase is defective.

a. If cohesin fails to form early in mitosis, the sister chromatids could separate prior to anaphase, resulting in improper segregation of chromosomes to daughter cells. b. If shugoshin is absent during meiosis, the cohesin at the centromere could be broken, allowing for the separation of sister chromatids during anaphase I. c. If shugoshin is not broken down, the cohesin at the centromere will remain protected from degradation. The intact cohesin will prevent the sister chromatids from separating during anaphase II of meiosis. d. If separase is defective, homologous chromosomes and sister chromatids will not separate in meiosis and mitosis, resulting in some cells that have too few chromosomes and some cells that have too many chromosomes.

Diploid cells have: a. two chromosomes b. two sets of chromosomes c. one set of chromosomes d. two pairs of homologous chromosomes

b.