A Comparison of Mitosis and Meiosis
Figure 13.9 summarizes
the key differences between meiosis and mitosis in diploid cells. Basically,
meiosis reduces the number of chromosome sets from two (diploid) to one
(haploid), whereas mitosis conserves the number of chromosome sets. Therefore,
meiosis produces cells that differ genetically from their parent cell and from
each other, whereas mitosisproduces daughter cells that are genetically
identical to their
parent
cell and to each other. Three events unique to meiosis occur during meiosis I:
Synapsis and crossing
over. During prophase I, duplicated
homologs pair up, and the
formation of the synaptonemal
complex between them holds
them in synapsis.
Crossing over also occurs
during prophase I. Synapsis and
crossing over normally do not
occur during prophase of
mitosis.
Homologous pairs at the
metaphase plate. At
metaphase I of meiosis,
chromosomes are positioned at the
metaphase plate as pairs of
homologs, rather than individual
chromosomes, as in metaphase
of mitosis.
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.
How
do sister chromatids stay together through meiosis I
but
separate from each other in meiosis II and mitosis? Sister chromatids are
attached along their lengths by protein complexes called cohesins. 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, one at the
start of anaphase I and one at anaphase II. In metaphase I, homologs are held
together by cohesion between sister chromatid arms in regions beyond points of
crossing over, where stretches of sister chromatids now belong to different
chromosomes. As shown in Figure 13.8, the combination of crossing over and
sister chromatid cohesion
along
the arms results in the formation of a chiasma. Chiasmata hold homologs
together as the spindle forms for the first meiotic division. 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 centromeres allows the sister chromatids to separate. Thus, sister
chromatid cohesion and crossing over, acting together, play an essential role in
the lining up of chromosomes by homologous pairs at metaphase I. Meiosis I is
called the reductional division because it halves the number of chromosome sets per cell—a
reduction from two sets (the diploid state) to one set (the haploid state).
During the second meiotic division, meiosis II (sometimes called the equational division), the
sister chromatids separate, producing haploid daughter cells. The mechanism for
separating sister chromatids is virtually identical in meiosis II and mitosis.
The molecular basis of chromosome behavior during meiosis continues to be a
focus of intense research.
