Concept 13.3 Meiosis reduces the number of chromosome sets from diploid to haploid Flashcards
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
meiosis I and meiosis II
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.
four daughter cells
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
four haploid daughter cells
Recall that sister chromatids are two copies of one chromosome, closely associated all along their lengths; this association is called
sister chromatid cohesion
Together, the sister chromatids make up one
duplicated chromosome
In contrast, the two chromosomes of a homologous pair are individual chromosomes that were
inherited from each parent.
Homologs appear alike in the microscope, but they may have different versions of genes at corresponding loci; each version is called an
allele of that gene
Homologs are not associated with each other in any obvious way except during
meiosis.
figure 13.7, animation overview of meiosis, figure 13.8, video meiosis I in sperm formation, bioflix animation meiosis, animation meiosis
figure 13.9
After interphase, the chromosomes have been duplicated and the sister chromatids are held together by proteins called
cohesins
- 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
allele of that gene on the other homolog.
The DNA of two nonsister chromatids—one maternal and one paternal—is broken by specific
proteins at precisely matching points.
2 Next, the formation of a zipper-like structure called the _______________________ holds one homolog tightly to the other.
synaptonemal complex
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.
synapsis
Thus, a __________________ is joined to a piece of maternal chromatid beyond the crossover point, and vice versa.
paternal chromatid
4 These points of crossing over become visible as _____________________ after the synaptonemal complex disassembles and the homologs move slightly apart from each other.
chiasmata (singular, chiasma)
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
sister chromatid cohesion
At least one crossover per chromosome must occur in order for the homologous pair to stay together as it moves to the
metaphase I plate
meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), whereas mitosis
conserves the number of chromosome sets.
meiosis produces cells that differ genetically from their parent cell and from each other, whereas mitosis produces
daughter cells that are genetically identical to their parent cell and to each other.
figure 13.10
Three events unique to meiosis occur during meiosis I:
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.
Sister chromatids stay together due to sister chromatid cohesion, mediated by
cohesin proteins
In mitosis, this attachment lasts until the end of metaphase, when enzymes cleave the cohesins, freeing the sister chromatids to move to
opposite poles of the cell.
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
one at the start of anaphase I and one at anaphase II.
The combination of crossing over and sister chromatid cohesion along the arms results in the formation of a
chiasma.
hold homologs together as the spindle forms for the first meiotic division
Chiasmata
At the onset of anaphase I, the release of cohesion along sister chromatid arms allows homologs to
separate
At anaphase II, the release of sister chromatid cohesion at the ______________ allows the sister chromatids to separate.
centromeres
Meiosis I reduces the number of chromosome sets: from
two (diploid) to one (haploid)
During the second meiotic division, sister chromatids separate, producing
haploid daughter cells
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