Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1008 Chapter 17: The Cell Cycle
Figure 17–57 A bivalent with three chiasmata resulting from three
crossover events. (A) Light micrograph of a grasshopper bivalent.
(B) Drawing showing the arrangement of the crossovers in (A). Note that
chromatid 1 has undergone an exchange with chromatid 3, and chromatid
2 has undergone exchanges with chromatids 3 and 4. Note also how
the combination of the chiasmata and the tight attachment of the sister
chromatid arms to each other (mediated by cohesin complexes) holds the
two homologs together after the synaptonemal complex has disassembled;
if either the chiasmata or the sister-chromatid cohesion failed to form, the
homologs would come apart at this stage and not be segregated properly in
meiosis I. (A, courtesy of Bernard John.)
Homolog Segregation Depends on Several Unique Features of
Meiosis I
A fundamental difference between meiosis I and mitosis (and meiosis II) is that
in meiosis I homologs rather than sister chromatids separate and then segregate
(see Figure 17–53). This difference depends on three features of meiosis I that distinguish
it from mitosis (Figure 17–58).
First, both sister kinetochores in a homolog must attach stably to the same
spindle pole. This type of attachment is normally avoided during mitosis (see Figure
17–33). In meiosis I, however, the two sister kinetochores are fused into a single
microtubule-binding unit that attaches to just one pole (see Figure 17–58A).
The fusion of sister kinetochores is achieved by a complex of proteins that is
(A)
(B)
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1 2 3
(A) MEIOSIS
kinetochore
microtubules
+
cohesin
kinetochore
centromere
cohesin
haploid daughter nuclei
kinetochore
chromatid
+
METAPHASE I
ANAPHASE I
METAPHASE II
ANAPHASE II
TELOPHASE II
(B)
MITOSIS
cohesin
chromatid
diploid daughter nuclei
centromere
+
kinetochore
microtubules
kinetochore
METAPHASE
ANAPHASE
TELOPHASE
Figure 17–58 Comparison of chromosome behavior in meiosis I, meiosis II, and mitosis. Chromosomes behave similarly in mitosis and meiosis
II, but they behave very differently in meiosis I. (A) In meiosis I, the two sister kinetochores are located side-by-side on each homolog and attach to
microtubules from the same spindle pole. The proteolytic cleavage of cohesin along the sister-chromatid arms unglues the arms and resolves the
crossovers, allowing the duplicated homologs to separate at anaphase I, while the residual cohesin at the centromeres keeps the sisters together.
Cleavage of centromeric cohesin allows the sister chromatids to separate at anaphase II. (B) In mitosis, by contrast, the two sister kinetochores
attach to microtubules from different spindle poles, and the two sister chromatids come apart at the start of anaphase and segregate into separate
daughter nuclei.
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