Essential Cell Biology 5th edition

Meiosis and Fertilization
661
(A)
(B)
metaphase
of meiosis II
anaphase
of meiosis II
kinetochore
centromere
COHESINS AT CENTROMERE
ARE DEGRADED; SISTER
CHROMATIDS SEPARATE
Figure 19−14 In meiosis II, as in mitosis,
the kinetochores on each sister chromatid
function independently, allowing the
two sister chromatids to be pulled to
opposite poles. (A) In metaphase of
meiosis II, the kinetochores of the sister
chromatids point in opposite directions.
(B) At anaphase of meiosis II, the cohesins
holding the sister chromatids together at
the centromere are degraded, allowing
kinetochore microtubules to pull the sister
chromatids to opposite poles.
versions of others (Figure 19−15A). This random assortment depends
solely on the way each bivalent happens to be positioned when it lines
up on the spindle during metaphase of meiosis I. Whether the maternal
or paternal homolog is captured by the microtubules from one pole or the
other depends on which way the bivalent is facing when the microtubules
connect to its kinetochore (see ECB5 Figure e19.14/19.14 19−13). Because the orientation of
each bivalent at the moment of capture is completely random, the assortment
of maternal and paternal chromosomes is random as well.
Thanks to this random reassortment of maternal and paternal homologs,
an individual could in principle produce 2 n genetically different gametes,
where n is the haploid number of chromosomes. With 23 chromosomes to
choose from, each human, for example, could in theory produce 2 23 —or
QUESTION 19–1
Why do you think that organisms do
not use the first steps of meiosis (up
to and including meiotic division I)
for the ordinary mitotic division of
somatic cells?
three pairs of
homologous chromosomes
one pair of
homologous chromosomes
maternal
paternal
maternal
paternal
INDEPENDENT ASSORTMENT
OF MATERNAL AND
PATERNAL HOMOLOGS
DURING MEIOSIS I
CROSSING-OVER
DURING MEIOTIC
PROPHASE
(A)
MEIOSIS II
possible gametes
(B)
MEIOTIC DIVISIONS
I AND II
possible gametes
Figure 19−15 Two kinds of genetic
reassortment generate new chromosome
combinations during meiosis. (A) The
independent assortment of the maternal
and paternal homologs during meiosis
produces 2 n different haploid gametes
for an organism with n chromosomes.
Here n = 3, and there are 2 3 , or 8,
different possible gametes. For simplicity,
chromosome crossing-over is not shown
here. (B) Crossing-over during meiotic
prophase exchanges segments of DNA
between homologous chromosomes and
thereby reassorts genes on each individual
chromosome. For simplicity, only a single
pair of homologous chromosomes is shown.
Both independent chromosome assortment
and crossing-over occur during every
meiosis.