Meiosis

Question 1

T/F. Meiosis consists of two successive divisions, the first and the second meiotic divisions, in w/c the DNA replicates both before meiosis 1 and 2.

Answer 1

False. DNA replicates only once before the first division.

Question 2

What are the five stages of prophase of the first meiotic division?

Answer 2

Leptotene (threadlike)
Zygotene (pairing)
Pachytene (thickening)
Diplotene (appearing double)
Diakinesis (further condensaiton)

Question 3

At this stage, each chromosome consists of a pair of sister chromatids. It starts with the first appearance of the chromosome.

Answer 3

Leptotene

Question 4

In this stage, homologous chromosomes pair to form bivalents.

Answer 4

Zygotene

Question 5

During the zygotene stage, homologous chromosomes pair and form what structure?

Answer 5

bivalents

Question 6

Leptotene, Diplotene, Anaphase I, Anaphase II gametes bound closely together by the ____.

Answer 6

synaptonemal complex

Question 7

The X and Y chromosomes undergo synapsis only at the distal end of both short arms in region called?

Answer 7

pseudoautosomal regions

Question 7

The precise mechanism by w/c homologous chromosomes pair in humans is unclear, but dispersed blocks of ____ are suspected to be involved in the initial alignment and protein complexes such as the ____ are involved.

Answer 7

repetitive DNA; synaptonemal complexes

Question 8

X and Y synapsis form _____, which is out of phase with the others and is condensed early in ____ as the sex body.

Answer 8

sex bivalents; pachytene

Question 9

The non - pairing, or differential, parts of the X and Y are transcriptionally silenced through a process called ____.

Answer 9

meiotic sex chromosome inactivation

Question 10

Meiotic sex chromosome inactivation is achieved by what process?

Answer 10

Phosphorylation of the nucleosomal histone

Question 11

T/F. The early condensation of the differential regions of X and Y chromosomes during meiotic sex chromosome inactivation is important in preventing crossing – over between the non - pairing regions.

Answer 11

True.

Question 12

_____ is the main stage of chromosomal thickening, and the pattern of chromosome condensation appears to correspond to the banding pattern seen at mitosis.

Answer 12

Pachytene

Question 13

During the pachytene stage, each chromosome is seen to consist of two chromatids; hence, each bivalent is a ____ of four strands.

Answer 13

tetrad

Question 14

Satellite association of the acrocentrics occurs at _____, due to the synapsis of homologous repetitive sequences (NOR) on non - homologous chromosomes.

Answer 14

pachytene

Question 15

During ____, the bivalents start to separate. Although the 2 chromosomes of each bivalent separate, the centromere of each remains intact, so the 2 chromatids of each chromosome remain together.

Answer 15

diplotene

Question 16

During longitudinal separation, the 2 members of each bivalent are seen to be in contact at several places, called _____.

Answer 16

chiasmata

Question 17

Chiasmata mark the location of ____, where the chromatids of homologous chromosomes have exchanged material in late pachytene

Answer 17

cross - overs

Question 18

On average, there are about _____ chiasmata per human male cell with at least one chiasma per chromosome arm.

Answer 18

52

Question 19

Short arms of which chromosomes do not form chiasmata?

Answer 19

acrocentric, and chromosome 18

Question 20

Characteristic appearance of chromosomes with chiasmata:

Answer 20

short chromosome with one chiasma - rod or cross
longer chromosomes with two chiasmata - ring
longer chromosomes with three chiasmata - figure 8

Question 21

At ____, the sex bivalent opens out and the X and Y chromosomes can be seen attached to one another by the tiny pairing segments at the ends of their short arms indicating homology of these regions.

Answer 21

diplotene

Question 22

Crossing over occurs only in what region of the sex chromosome?

Answer 22

pseudoautosomal region 1 (PAR1) in the tip of and X and Y short arm

Question 23

T/F. Sequences in the PAR region show sex-linked rather than autosomal inheritance.

Answer 23

False. autosomal rather than sex-linked

Question 24

____ is the final stage of prophase, during which the chromosomes coil more tightly and so stain more deeply.

Answer 24

Diakenesis

Question 24

This region present in the tip of the long arm of chromosome X possesses a much lower rate of recombination and is not usually apparent at diakenesis.

Answer 24

PAR2

Question 24

____ begins when the nuclear membrane disappears & the chromosomes move to the equatorial plane.

Answer 24

Metaphase

Question 25

At _____, the two members of each bivalent separate, one going and sister chromatids pass to opposite poles.

Answer 25

anaphase

Question 26

T/F. The second meiotic division chromosomes are rather less coiled than mitotic ones and show splaying of the chromatids.

Answer 26

False. More coiled.

Question 27

T/F. Meiosis is suspended until puberty in females.

Answer 27

True.

Question 28

As the chromosomes assort independently during meiosis, this results in ____ different possible combinations of chromosomes in the gametes from each parent. Hence, there are ____ possible combinations in the zygote.

Answer 28

2^23 or 8,388,608; 2^46

Question 29

There is still further scope for variation provided by crossing - over during meiosis. If there is, on average, only one cross - over per chromosome & a 10% paternal/maternal allele difference, then the number of possible zygotes exceeds ____.

Answer 29

6 × 10^43 .

Question 30

Three important consequence of meiosis:

Answer 30

1. Gametes contain only one representative of each homologous pair of chromosomes.
2. There is random assortment of paternal and maternal homologues.
3. Crossing - over ensures uniqueness by further increasing genetic variation.