Essential Cell Biology 5th edition

660 CHAPTER 19 Sexual Reproduction and Genetics
Figure 19−13 Chiasmata help ensure
proper segregation of duplicated
homologs during the first meiotic
division. (A) In metaphase of meiosis I,
chiasmata created by crossing-over hold the
maternal and paternal homologs together.
At this stage, cohesin proteins keep the
sister chromatids glued together along
their entire length (see Figure 19–10). The
kinetochores of sister chromatids function as
a single unit in meiosis I, and microtubules
that attach to them point toward the same
spindle pole. (B) At anaphase of meiosis I,
the cohesins holding the arms of the
sister chromatids together are suddenly
degraded, allowing the homologs to be
separated. Cohesins at the centromere
continue to hold the sister chromatids
together as the homologs are pulled apart.
(A)
(B)
metaphase
of meiosis I
kinetochores
on the two
sister chromatids
function as
a single unit
anaphase
of meiosis I
chiasma
kinetochore microtubules
attached to sister
chromatids point in same
direction
ARMS OF SISTER CHROMATIDS
BECOME UNGLUED, ALLOWING THE
DUPLICATED HOMOLOGS TO SEPARATE
chiasmata resist this pulling (Figure 19–13A). In so doing, the chiasmata
help to position and stabilize bivalents at the metaphase plate.
In addition to the chiasmata, which hold the maternal and paternal
homologs together, cohesin proteins (described in Chapter 18) keep the
sister chromatids glued together along their entire length at meiosis I
(see Figure 19−10). At the start of anaphase I, the cohesin proteins that
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hold the chromosome arms together are suddenly degraded. This release
allows the arms to separate and the recombined homologs to be pulled
apart (Figure 19−13B). If the arms were not released in this way, the
duplicated maternal and paternal homologs would remain tethered to
one another by the homologous DNA segments they had exchanged and
would not separate during anaphase I.
The Second Meiotic Division Produces Haploid
Daughter Nuclei
To separate the sister chromatids and produce cells with a haploid
amount of DNA, a second round of division, meiosis II, follows soon after
the first—without further DNA replication or any significant interphase
period. A meiotic spindle forms, and the kinetochores on each pair of
sister chromatids now attach to kinetochore microtubules that point in
opposite directions, as they would in an ordinary mitotic division. At anaphase
of meiosis II, the remaining, meiosis-specific cohesins—located at
the centromere—are degraded, and the sister chromatids are pulled apart
(Figure 19−14). The entire process is shown in Movie 19.1.
Haploid Gametes Contain Reassorted Genetic
Information
Even though they share the same parents, no two siblings are genetically
the same (unless they are identical twins). These genetic differences
are initiated long before sperm meets egg, when meiosis I produces two
kinds of randomizing genetic reassortment.
First, as we have seen, the maternal and paternal chromosomes are shuffled
and dealt out randomly during meiosis I. Although the chromosomes
are carefully distributed so that each gamete receives one and only one
copy of each chromosome, the choice between the maternal or paternal
homolog is made by chance, like the flip of a coin. Thus, each gamete
contains the maternal versions of some chromosomes and the paternal