Cell Nucleus Biogenesis, Structure and Function
391
condensation along the chromosome axis.
Chromosome condensation occurs during
prophase and continues for about 30 min;
as this process nears completion the nu-
clear membrane breaks down, marking the
transition from interphase to mitosis. By
late prophase, the chromosomes are fully
condensed and are interacting with the mi-
totic microtubules. Microtubules function
to adjust the position of the chromosomes
until they are all aligned, at metaphase,
across the center of the cell.
Chromosome condensation generates
an elongated structure, which ensures
that the kinetochores located within the
centromeres of each chromosome are
accessible to the microtubules that will
eventually drag each daughter chromatid
to the site where the two daughter nuclei
will form. As the efFcacy of this process is
critical to the maintenance of genomic in-
tegrity, a mitotic checkpoint ensures that
the chromatids are only able to separate
once all kinetochores are attached via mi-
crotubules to the spindle. Prior to this
point, a G2 checkpoint veriFes that the
replication process is complete. Once the
kinetochores are fully engaged, the cohesin
proteins that hold the sister chromatids to-
gether are released and the chromatids
move from the metaphase plate toward
the spindle poles; prior to this, topoiso-
merase II serves to resolve interlocked
DNA loops that arise during replication.
Microtubule and kinetochore-associated
motor proteins, together with the dynamic
properties of the microtubules themselves,
provide the motive forces that reposition
the daughter chromatids within the cell.
The mitotic phase that involves migration
of sister chromatids toward the spindle
poles is called
anaphase
andth
i
si
sfo
l
-
lowed by
telophase
, where chromosome
decondensation begins. At this stage, the
mitotic cell cycle machinery is destroyed
and the new daughter cells form by cytoki-
nesis.
2.3.2
Meiosis
In almost all cells, proliferation involves
the duplication of a diploid chromosome
set to form a tetraploid set, which is then
separated, at mitosis, to form daughter
cells that are once more diploid. Meio-
sis is a related process, which occurs in
germ cells and serves to reduce the level
of ploidy, so that on completing meiosis
cells contain a single, haploid, chromo-
some set. Meiosis is a complex process,
which not only reduces cell ploidy but also
allows DNA exchange between homolo-
gous chromosomes to stimulate genetic
diversity. Perhaps the easiest way to un-
derstand meiosis is to imagine the process
as two consecutive mitotic events without
an intervening S-phase. The cells that un-
dertake meiosis are tetraploid. The Frst
series of events – called
meiotic division
one
– follow the same principle as mito-
sis but include a much longer prophase
during which chromosome pairing and re-
combination take place; in human testes
the duration of meiosis is roughly 25 days.
The Frst meiotic prophase – during which
time the nucleus remains intact – is tradi-
tionally divided into Fve sequential stages:
leptotene, zygotene, pachytene, diplotene,
and diakinesis. Homologous chromosome
pairs synapse during zygotene and dur-
ing pachytene, the resulting synaptonemal
complex is thought to allow homologous
recombination of the paired chromosomes
to occur. The molecular details of this
recombination process are poorly un-
derstood. The recombination complexes
dissolve during diplotene to leave ho-
mologs that are attached at a small number
of chiasma. The Frst meiotic division
then proceeds through metaphase and
anaphase and daughter nuclei are formed.
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