Chapter 5: Cell division

genetic recombination

exchange of DNA between chromosomes to produce genetically diverse offspring.

Synapsis

the pairing up of homologous chromosomes to form tetrads (aka bivalents).

which stages are most similar during cell division?

the stages of mitosis and meiosis II are all similar

chiasmata

where two chromosomes of a homologous pair cross over during synapsis, causing genetic recombination.

how many number of chromosomes and chromatids are in prophase II, metaphase II, anaphase II, and telophase II?

p2: 23 chromosomes, 46 chromatids m2: 23 chromosomes, 46 chromatids a2: 46 chromosomes, 46 chromatids t2: 46 chromosomes, 46 chromatids

at what stage of meiosis does synapsis occur?

prophase I Synapsis is the pairing of two homologous chromosomes that occurs during prophase I of meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them.

Synaptonemal complex

protein structure that forms between homologous chromosomes during synapsis.

Which of the following events occurs during mitosis but does NOT occur during meiosis I?

splitting of centromeres During both mitosis and meiosis I, the nuclear membrane is broken down, and cytokinesis occurs at the end to produce daughter cells. Synapsis, or the pairing of homologous chromosomes, is a process that takes place only in meiosis. Splitting of the centromeres occurs only in mitosis, and not in meiosis I. Splitting of centromeres, however, does occur during Anaphase II of meiosis II.

After meiosis I...

- homologous chromosomes split. - The diploid cell has become HAPLOID. - the chromosome number is 23 and the chromatid number is 46. - the chromosomes are composed of two chromatids

After meiosis II...

- sisters chromatids split - each daughter cell is haploid - the chromosome number is 23 and the chromatid number is 23 - the chromosomes are composed of one chromatid.

during mitosis

- start off with a diploid cell (2n, 46 chromosomes) and end up with haploid(n,23 chromosomes) in Meiosis I and II.

how many number of chromosomes and chromatids are in end of meiosis II (separated cells)?

23 chromosomes, 23 chromatids

how many number of chromosomes and chromatids are in end of meiosis I (separated cells)?

23 chromosomes, 46 chromatids

A tiger (2N = 40) mates with a lion (2N = 38), producing a hybrid offspring known as a liger. What is the makeup of the liger offspring?

2N = 39 Recall that a diploid offspring in sexually reproducing species is formed by the fusion of haploid gametes. We are given the ploidy (diploid here) and chromosome count for both the tiger and lion. Since the haploid gametes are formed by meiosis, they must have half the chromosome count of the somatic cells. The tiger gametes must have 20 chromosomes while the lion gametes must have 19 chromosomes. The fusion of male and female gametes from these species will result in a diploid zygote with a total of 39 chromosomes from 20+19.

A diploid organism has a haploid chromosome number n=15. How many chromatids would you expect to find during metaphase II of meiosis in one of the daughter cells?

30 If haploid number (n) is equal to 15, the diploid (2n) number is equal to 30.

how many number of chromosomes and chromatids are in end of mitosis (separated cells)?

46 chromosomes, 46 chromatids

What is the first event of meiosis? A. Tetrads line up at the midplate of the cell B. Homologous chromosomes pair up and cross over C. DNA is replicated D. Homologous chromosomes are segregated to opposite poles

B. Homologous chromosomes pair up and cross over The first event of meiosis is prophase I. In this phase, chromatin condense into chromosomes. The nucleolus and nuclear envelope will disappear. And homologous chromosomes pair up and crossing over occurs.

An organism is 2N = 16. What will the chromosome and chromatid count in each cell after Meiosis I has completed?

8 and 16 Looking at meiosis, during meiosis I, only the homologous chromosomes are separated so the daughter cells have half the chromosome count of the parent cell. However, since each chromosome consists of two sister chromatids, the chromatid count in each daughter cell is half of the parent. During meiosis II, sister chromatids are separated. Once they are separated, they can be considered individual chromosomes so the chromosome count will double during anaphase II. At the end of meiosis, after cytokinesis II, we have haploid gametes with half the chromosome count as the parent and since chromatids were separated, each chromosome consists of one chromatid. We know that 2N = 16 so after replication, the chromosome count is 16 and the chromatid count is 32. We know that meiosis I separates homologous chromosomes. Thus, each daughter cell after meiosis I will have half the chromosomes as the parent cell, 8 chromosomes. And since sister chromatids have not yet been separated, we know that each daughter cell has 16 chromatids on 8 chromosomes.

Interphase

90% of the cell cycle happens during interphase G1 phase - cell grows in preparation for cell division. If favorable conditions for division, the cell will enter the S phase. If unfavorable, the cell will enter the G0 phase. G0 phase - cells still carry out their functions but halt in the cell cycle. Cells that do not divide are stuck here. S phase - cell replicates its genome. CENTROSOMES are also duplicated here. G2 phase - the cell continues to grow and prepare for cell division by checking DNA for any errors after replication. It also checks for maturation promoting factor (MPF, also known as mitosis promoting factor) which needs to be present in adequate amounts for cell cycle continuation. Organelles are also replicated during this stage (NOT CENTROSOMES).

All of the following statements about mitosis and meiosis are correct EXCEPT one. Which is the EXCEPTION? A. Sister chromatids are separated in mitosis during anaphase and in meiosis during anaphase I B. Both mitosis and meiosis occurs in germ cells C. Independent assortment only occurs in meiosis D. Identical daughter cells after cell division is only a feature of mitosis E. In humans, the chromosome number after meiosis I is 23 and after mitosis is 46

A. Sister chromatids are separated in mitosis during anaphase and in meiosis during anaphase I The first part of the statement is true: sister chromatids are split in mitosis in anaphase. The second part of the statement is false: sister chromatids are split during anaphase II of meiosis. During meiosis I, homologous chromosomes are split. During meiosis II, sister chromatids are split.

Which of the following is true of a pair of homologous chromosomes? A. Contain different genes B. Contain distinct alleles C. Separate during anaphase II of meiosis D. Connected by a centromere E. Differ in size

B. Contain distinct alleles Homologous chromosomes are the two 'different copies' of the same chromosome in a diploid organism. One copy is given by the mother and the other copy is given by the father. While homologous chromosomes contain the same genes, they can have slight variations that are known as alleles. C. Separate during anaphase II of meiosis Sister chromatids separate during anaphase II. Homologous chromosomes separate during anaphase I. D. Connected by a centromere Sister chromatids, not homologous chromosomes, are connected by centromeres.

After meiosis I has completed, each daughter cell is A. haploid, and the chromosomes are composed of one chromatid. B. haploid, and the chromosomes are composed of two chromatids. C. diploid, and the chromosomes are composed of one chromatid. D. diploid, and the chromosomes are composed of two chromatids.

B. haploid, and the chromosomes are composed of two chromatids. After meiosis I, homologous chromosomes split; each daughter cell is haploid, and the chromosomes are composed of two chromatids. After meiosis II, sisters chromatids split; each daughter cell is haploid, and the chromosomes are composed of one chromatid.

Which structure forms to separate a dividing prokaryote into two separate cells? A. Cell plate B. Vesicle C. Cleavage furrow D. Septum E. Metaphase plate

Binary fission is a form of asexual reproduction that occurs in prokaryotes and some organelles, such as mitochondria and chloroplasts. Binary fission begins as one parent chromosome replicates into two daughter chromosomes. Then, the chromosomes move to opposite ends of the cell before a SEPTUM separates them and partitions the parent cell into two daughter cells, which then split into separate cells.

All of the following occur during G2 EXCEPT one. Which is the EXCEPTION? A. Genome is checked for DNA errors B. Cell is checked for adequate amounts of maturation promoting factor C. Centrosome are replicated D. Organelles are replicated

C. Centrosome are replicated Centrosomes are duplicated during the S phase, not the G2 phase.

Which of the following is most likely to lead to uncontrolled cell proliferation? A. Entry to the G0 phase of the cell cycle B. Inhibited cyclin dependent kinase C. Downregulated p53 gene D. Loss-of-function mutations of proto-oncogenes

C. Downregulated p53 gene Tumor suppressor genes are genes that regulate a cell during cell division and replication. When a tumor suppressor gene is mutated or downregulated, it results in a loss or reduction in its function. Keep in mind that loss-of-function mutations in both copies of these genes are required for cancer causing, null tumor suppressor alleles. For example, p53 is a tumor suppressor gene that is known as the 'guardian of the cell'. The downregulation of p53 leads to an increase in the frequency of cell division.

How many autosomes are present in the nucleus of a human cell? A. 2 B. 22 C. 23 D. 44 E. 46

D. 44 The X and Y chromosomes are known as the sex chromosomes. Non-sex chromosomes are called autosomes. There are 46 total chromosomes in humans. Two of them are sex chromosomes. The rest are autosomes, therefore there are 44 autosomes in each body cell, and two sex chromosomes (XX or XY).

During which phase of the cell cycle does the exchange of genetic information occur? A. Immediately after replication of chromosomes - Telophase I B. Before nuclear envelope disintegrates - Anaphase II C. During formation of chiasmata - Prophase I D. Before condensation of chromosomes - Prophase II E. Before the tetrads are formed - Anaphase I

During formation of chiasma - Prophase I Let us focus on the early portion of meiosis I where genetic recombination occurs, Prophase I. First, the nuclear envelope disintegrates while the chromatin condensed into chromosomes. Each chromosome will consist of two sister chromatids which are still joined to each from replication. Next, homologous chromosomes, chromosomes that carry the same genes, can CROSS OVER when one sister chromatid from each physically crosses over with the other. This crossing over physically links the two and is known as synapsis and the result is known as a tetrad due to the presence of four sister chromatids. The specific site of crossing over is known as the chiasma. Later, during anaphase I when homologous chromosomes are pulled apart, some genetic material is exchanged between the sister chromatids that formed the chiasma. From this, we can see that as crossing over occurs, the chiasma is formed. Therefore, D is the correct answer.

If 2n=46, how many tetrads are present during metaphase I?

During prophase I of meiosis, homologous chromosomes pair up to form tetrads, allowing for crossing over to take place. If the total number of chromosomes in a cell is 46, and the cell is diploid (2n), then 23 homologous pairs would line up in the form of tetrads. Remember that ONE TETRAD consists of TWO CHROMOSOMES (or four chromatids) which have undergone synapsis to pair up, allowing for crossing over to occur at regions called chiasmata. So during metaphase I, there are 46 chromosomes meaning there are 23 tetrads IN TOTAL

A researcher introduces a chemical into a tumor cell that has begun to replicate out of control. After introducing the chemical, the formation of microtubules is stabilized and they are no longer able to shorten. What phase of the cell cycle has been disrupted?

M phase Mitosis is most affected by microtubule inhibiting chemicals because the microtubule-organizing centers are directly involved in moving chromosomes around the eukaryotic cell.

if a zebra is 2n = 18 and a horse is 2n = 24, what is the genetic makeup of the hybrid offspring?

The hybrid will receive 9 chromosomes from the zebra parent, and 12 chromosomes from the horse parent. This will combine to form 21 chromosomes in the hybrid offspring.

A diploid cell replicates via mitosis resulting in two daughter cells. How are the daughter cells related? A. Genetically identical to the parent cell but not to each other B. Both are haploid C. They are both genetically identical to the original parent D. Contain different chromosomes from each other E. Are genetically identical to each other but not to the original parent

They are both genetically identical to the original parent Mitosis is a form of cell replication in which a cell replicates its DNA. The DNA is then separated into two new nuclei in two new daughter cells. Since all of the DNA is replicated, the daughter cells will be genetically identical to the parent cell.

homologous chromosomes

contain the same genes, but slightly different versions (alleles) of those genes. are the two 'different copies' of the same chromosome in a diploid organism. One copy is given by the mother and the other copy is given by the father.

binary fission

form of asexual reproduction that occurs in prokaryotes and some organelles, such as mitochondria and chloroplasts. Binary fission begins as one parent chromosome replicates into two daughter chromosomes. Then, the chromosomes move to opposite ends of the cell before a septum separates them and partitions the parent cell into two daughter cells, which then split into separate cells.

cleavage furrow

forms in eukaryotic animal cells to separate the cytoplasm of the two daughter cells through cytokinesis.

homologous chromosomes are separated during which stage?

meiosis I (anaphase I)

sister chromatids are separated after which stage?

meiosis II

how many number of chromosomes and chromatids are in mitotic prophase, metaphase, anaphase, and telophase?

mp: 46 chromosomes, 92 chromatids mm: 46 chromosomes, 92 chromatids ma: 92 chromosomes, 92 chromatids mt: 92 chromosomes, 92 chromatids

mitotic prophase, mitotic metaphase, mitotic anaphase, mitotic telophase

mp: no crossing over, nuclear envelope disappears mm: chromosomes line up single file ma: sister chromatids separate mt: nuclear envelope reappears

prophase 2, metaphase 2, anaphase 2, telophase 2

mp: no crossing over, nuclear envelope disappears mm: chromosomes line up single file ma: sister chromatids separate mt: nuclear envelope reappears

how many number of chromosomes and chromatids are in prophase I, metaphase I, anaphase I, and telophase I?

p1: 46 chromosomes, 92 chromatids m1: 46 chromosomes, 92 chromatids a1: 46 chromosomes, 92 chromatids t1: 46 chromosomes, 92 chromatids

prophase 1, metaphase 1, anaphase 1, telophase 1

p1: Crossing over occurs, nuclear envelope disappears m1: Homologous pairs randomly line up in double file a1: Homologous chromosomes separate t1: Nuclear envelope reappears

tetrads (bivalents)

pair of two homologous chromosomes each with two sister chromatids.