Concept 13.3 Meiosis reduces the number of chromosome sets from diploid to haploid

Question 1

Meiosis, like mitosis, is preceded by the duplication of chromosomes. However, this single duplication is followed by not one but two consecutive cell divisions, called

Answer 1

meiosis I and meiosis II

Question 2

These two divisions result in _________________ (rather than the two daughter cells of mitosis), each with only half as many chromosomes as the parent cell—one set, rather than two.

Answer 2

four daughter cells

Question 3

The overview of meiosis in Figure 13.7 shows, for a single pair of homologous chromosomes in a diploid cell, that both members of the pair are duplicated and the copies sorted into

Answer 3

four haploid daughter cells

Question 4

Recall that sister chromatids are two copies of one chromosome, closely associated all along their lengths; this association is called

Answer 4

sister chromatid cohesion

Question 5

Together, the sister chromatids make up one

Answer 5

duplicated chromosome

Question 6

In contrast, the two chromosomes of a homologous pair are individual chromosomes that were

Answer 6

inherited from each parent.

Question 7

Homologs appear alike in the microscope, but they may have different versions of genes at corresponding loci; each version is called an

Answer 7

allele of that gene

Question 8

Homologs are not associated with each other in any obvious way except during

Answer 8

meiosis.

Question 9

figure 13.7, animation overview of meiosis, figure 13.8, video meiosis I in sperm formation, bioflix animation meiosis, animation meiosis

Question 10

figure 13.9

Question 11

After interphase, the chromosomes have been duplicated and the sister chromatids are held together by proteins called

Answer 11

cohesins

Question 12

1. Early in prophase I, the two members of a homologous pair associate loosely along their length. Each gene on one homolog is aligned precisely with the corresponding

Answer 12

allele of that gene on the other homolog.

Question 13

The DNA of two nonsister chromatids—one maternal and one paternal—is broken by specific

Answer 13

proteins at precisely matching points.

Question 14

2 Next, the formation of a zipper-like structure called the _______________________ holds one homolog tightly to the other.

Answer 14

synaptonemal complex

Question 15

3 During this association, called ____________, the DNA breaks are closed up so that each broken end is joined to the corresponding segment of the nonsister chromatid.

Answer 15

synapsis

Question 16

Thus, a __________________ is joined to a piece of maternal chromatid beyond the crossover point, and vice versa.

Answer 16

paternal chromatid

Question 17

4 These points of crossing over become visible as _____________________ after the synaptonemal complex disassembles and the homologs move slightly apart from each other.

Answer 17

chiasmata (singular, chiasma)

Question 18

The homologs remain attached because sister chromatids are still held together by _______________________, even though some of the DNA may no longer be attached to its original chromosome

Answer 18

sister chromatid cohesion

Question 19

At least one crossover per chromosome must occur in order for the homologous pair to stay together as it moves to the

Answer 19

metaphase I plate

Question 20

meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), whereas mitosis

Answer 20

conserves the number of chromosome sets.

Question 21

meiosis produces cells that differ genetically from their parent cell and from each other, whereas mitosis produces

Answer 21

daughter cells that are genetically identical to their parent cell and to each other.

Question 22

figure 13.10

Question 23

Three events unique to meiosis occur during meiosis I:

Answer 23

1.Synapsis and crossing over. During prophase I, duplicated homologs pair up and crossing over occurs
2.Alignment of homologous pairs at the metaphase plate. At metaphase I of meiosis, pairs of homologs are positioned at the metaphase plate, rather than individual chromosomes, as in metaphase of mitosis.
3.Separation of homologs. At anaphase I of meiosis, the duplicated chromosomes of each homologous pair move toward opposite poles, but the sister chromatids of each duplicated chromosome remain attached. In anaphase of mitosis, by contrast, sister chromatids separate.

Question 24

Sister chromatids stay together due to sister chromatid cohesion, mediated by

Answer 24

cohesin proteins

Question 25

In mitosis, this attachment lasts until the end of metaphase, when enzymes cleave the cohesins, freeing the sister chromatids to move to

Answer 25

opposite poles of the cell.

Question 26

In meiosis, sister chromatid cohesion is released in two steps, _________________________________________ In metaphase I, the two homologs of each pair are held together because there is still cohesion between sister chromatid arms in regions beyond points of crossing over, where stretches of sister chromatids now belong to different chromosomes

Answer 26

one at the start of anaphase I and one at anaphase II.

Question 27

The combination of crossing over and sister chromatid cohesion along the arms results in the formation of a

Answer 27

chiasma.

Question 28

hold homologs together as the spindle forms for the first meiotic division

Answer 28

Chiasmata

Question 29

At the onset of anaphase I, the release of cohesion along sister chromatid arms allows homologs to

Answer 29

separate

Question 30

At anaphase II, the release of sister chromatid cohesion at the ______________ allows the sister chromatids to separate.

Answer 30

centromeres

Question 31

Meiosis I reduces the number of chromosome sets: from

Answer 31

two (diploid) to one (haploid)

Question 32

During the second meiotic division, sister chromatids separate, producing

Answer 32

haploid daughter cells

Question 33

scientific skills exercise