Top1-Ch2-P24-42End

Question 1

Number of chromosomes and number of DNA molecules during cell cycle picture?

Answer 1

Question 2

Genetic consequences of cell cycle? Name three points.

Answer 2

1. One cell to two cells with same genetic information.
2. Two cells produced contain a full complement of chromosomes.
3. Not all cells produced have identical cytoplasmic content.

Question 3

Name the two processes of sexual reproduction?

Answer 3

1. Meiosis - leads to gametes in which chromosome number is reduced by half.
2. Fertilisation - where two haploid gametes fuse and restore chromosome number to its original diploid value.

Gametes are reproductive cells that unite during sexual reproduction to form a new cell called a zygote.

Question 4

How does meiosis differ from mitosis?

Answer 4

• Mitosis single nuclear division and usually a single cell division. Meiosis has two divisions.
• After mitosis chromosome number in new cells is same as original cell. Meiosis chromosome numbers in new cells are halved.
• Mitosis produces genetically identical cells but mieosis produces genetically variable cells.

Question 5

Name the phases of Meiosis?

Answer 5

Like mitosis, meiosis is preceded by a interphase stage that includes G1, S, and G2 phases.

Meiosis I

Prophase I

Metaphase I

Anaphase I

Telophase I

Cytokinesis

Interkinesis

Meiosis II

Prophase II

Metaphase II

Anaphase II

Telophase II

Cytokinesis

Question 6

Meiosis Prophase I is divided into how many substages? Explain a bit about each.

Answer 6

Five:

Leptotene - chromosomes contract and become visible.

Zygotene - chromosomes continue to condense. Homologous chromosomes pair up and begin synapsis, a very close pairing association. Each homologous pair of synapsed chromosomes consists of four chromatids called a bivalent or tetrad.

Pachytene - Chromosomes become shorter and thicker and a three part synaptonemal complex develops between homologous chromosomes.

Diplotene - centromeres of the paired chromosomes move apart. The two homologs remain attached at each chiasma which is the result of crossing over

Diakinesis - chromsome condensation continues and the chiasmata move towards the ends of the chromosomes as the strands slip apart so the homologs remain paired only at the tips.

Near the end of prophase I the nuclear membrane breaks down and the spindle forms setting the stage for metaphase I

Define: chiasmata - a point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands.

Crossing over occurs during prophase I in which homologous chromosomes exchange genetic information. This generates genetic variation and is essential for the proper alignment and separation of homologous chromosomes.

Question 7

Meiosis metaphase I is?

Answer 7

when homologous pairs of chromosomes align along the metaphase plate. Microtubules from each pole attach to different chromosomes.

Question 8

Meiosis anaphase I is?

Answer 8

separation of homologous chromosomes. Two chromosomes of homologous pair are pulled towards opposite poles.

Question 9

meiosis telophase I is?

Answer 9

when the chromosomes arrive at the spindle poles and the cytoplasm divides.

Question 10

meiosis, interkinesis is?

Answer 10

Period between Meiosis I and Meiosis II in which the nuclear membrane re-forms around the chromosomes clustered at each pole, the spindle breaks down and the chromosomes relax.

Question 11

meiosis, prophase II is?

Answer 11

when events of interkinesis are reversed. chromosomes recondense, spindle re-forms and nuclear envelope breaks down.

Some cell types chromosomes remain condensed in interkinesis. These cells move directly from cytokinesis into metaphase II.

Question 12

meiosis, metaphase II is?

Answer 12

chromosomes line up on the metaphase plate.

Question 13

meiosis, anaphase II is ?

Answer 13

when the kintechores of the sister chromatids separate and the chromatids are pulled to opposite poles. Each chromatid is now a distinct chromosome.

Question 14

meiosis, telophase II is ?

Answer 14

chromosomes arrive at spindle poles, nuclear envelope reforms around the chromosomes and cytoplasm divides.

Question 15

meiosis, cytokinesis in Meiosis II is when?

Answer 15

cytoplasm divides.

Question 16

meiosis, cytokinesis in Meiosis I is when?

Answer 16

the cytoplasm divides to produce two cells, each having half the original number of chromosomes.

Question 17

Meiosis graphic

Answer 17

Question 18

Crossing over in meiosis is?

Answer 18

After each homologous chromosome replicates one chromosome has AB (chromatids are the same) and the other has ab. Then one part of one chromatid from each chromosome crosses over with the same spot from the other chromosome. So, one chromosome has two different chromatids. Ie AB, aB and the other chromosome has Ab, ab. So each chromatid is now different. This shuffles alleles on the same chromosome into new combinations.

Question 19

Explain “Random Separation of Homologous Chromosomes”

Answer 19

Happens in metaphase I after random alignment of chromosomes in metaphase I. One chromosome in each homologous pair is maternal and the other paternal in origin. But when the line up for the homologous pairs to be separated they line up randomly. So, maternals and paternals can go to either side, with maternals and paternals mixing. This shuffles alleles on different chromosomes into new combinations.

Question 20

Genetic differences among cells result from two processes that are unique to meiosis. What are the two processes?

Answer 20

1. crossing over
2. random separation of homologous chromosomes

Question 21

Compare differences between mitosis and meiosis

Answer 21

Question 22

Cohesin is?

Answer 22

a protein that holds the chromatids together during meiosis and mitosis. Keeps sister chromatids together and during cross over also tethers homologs together at their ends.

In Anaphase I cohesin along chromosome arm is broken pulling two homologs apart but shugoshin prevents the two sister chromatids from separating.

To learn: cohesin and shugoshin

Question 23

Meiosis in male animals explain.

Answer 23

Diploid primordial germ cells divide mitotically to produce diploid cells called spermatogonia.

Each spermatagonium can undergo repeated rounds of mitosis producing more. Or, it can enter into prophase I and is called a primary spermatocyte (still diploid).

Primary spermatocyte completes meiosis I, producing two haploid secondary spermatocytes that then undergo meisos II, producing two haploid spermatids.

Therefore, each primary spermatocyte produces a total of four haploid spermatids which mature and develop into sperm.

Question 24

Spermatogenesis is?

Answer 24

is the production of gametes in a male animal and occurs in the testes.

Question 25

What is formula to calculate how many different combinations of gametes can be formed by pairs of chromosomes?

Answer 25

2ⁿ, where n equals the number of homologous pairs. Ie three homologous pairs produce eight gametes.

Question 26

Oogenesis is?

Answer 26

the production of gametes in a female animal

Question 27

Meiosis in female animals explain.

Answer 27

Within ovaries diploid primordial germ cells divide mitotically to produce oogonia which can undergo repeated mitosis or go into meiosis.

Enter prophase I and are called primary oocytes, which completes meiosis I and divides.

During cytokinesis most cytoplasm goes to one of the two haploid cells called the secondary oocyte. The smaller cell (first polar body) may or may not divide further or may disintegrate.

Secondary oocyte completes meiosis II and cytokinesis is again unequal. The larger cell is ovum (mature female gamete) and smaller cell is second polar body. Only the ovum can be fertilised and the polar bodies usually disintegrate.

Question 28

Apart from basic explanation of animal meiosis and differences between male and femal, how else does oogenesis differ from spermatogenesis in mammals?

Answer 28

Sperm formation occurs continuously during male adult reproductive life.

For females, oogenesis begins before birth giving rise to primary oocytes. Meiosis is then stalled in prophase I. Suspended animation, in humans, may last 30-40 years.

Before ovulation, rising hormones, stimulate one or more primary oocytes to recommence meiosis. Meiosis I is completed and a secondary oocyte is ovulated from the ovary.

In humans and many other species, Meiosis II is delayed until contact with sperm. When sperm penetrates outer layer of secondary oocyte Meiosis II occurs. Second polar body is extruded (forced out) from egg and nuclei of sperm and newly formed ovum fuse to form zygote.