Biology Reference
In-Depth Information
signal-transduction pathways whose effectors interact with the cyclin/cyclin-
dependent protein kinases to block the cell cycle. Blocking the cell cycle allows
time for repair of damage at G
1
(before DNA replication) or just before mitosis
at the G
2
DNA-damage checkpoint.
Chromosome replication takes place during the S phase, and the duplicated
chromosomes remain physically connected (as sister chromatids) until anaphase
of mitosis. The cohesion of the sister chromatids is what permits chromosome
segregation to take place long after chromosome duplication, and this cohe-
sion is due to a multisubunit complex called cohesin. Cleavage of one of cohe-
sin's subunits seems to trigger separation of the sister chromatids at the onset
of anaphase (
Nasmyth et al. 2000
). The ability of eukaryotic cells to delay seg-
regation of the replicated chromosomes until long after they have been dupli-
cated distinguishes the eukaryotic cell cycle from that of bacteria, in which
chromosome segregation starts immediately after DNA replication is initiated.
The separation of chromosome duplication and segregation has played a cen-
tral role in the evolution of eukaryotic organisms (
Nasmyth et al. 2000
). Mitotic
chromosome condensation, without which large genomes cannot be partitioned
between daughter cells at cell division, would not be possible if chromosome
segregation coincided with DNA replication. The gap (G
2
) between S and M
phases in the cell cycle thus makes possible the evolution of large genomes that
can be transmitted safely to daughter cells.
3.12.2 Meiosis
Meiosis probably evolved from a mitosis-like process (
van Heemst and Heytig
2000
). Meiosis is responsible for two essential aspects of the sexual life cycle in
eukaryotes: the transition from the diploid to the haploid state and the genera-
tion of new combinations of alleles. Meiosis occurs only in the germ line (ovaries
or testes).
During
meiosis
, cells are produced that have a reduced number of chromo-
somes (the haploid or n number). This reduction means that when the germ
cells (eggs and sperm) fuse, the diploid (2n) number of chromosomes is restored
(
Figure 3.4
). If meiosis did not reduce the number of chromosomes to n, the
number of chromosomes in a sexually reproducing organism would double each
generation. Both divisions in meiosis have prophase, metaphase, anaphase,
and telophase stages, but their details are different (
Figure 3.4
). Meiosis may
require days or weeks (or longer) to complete. The essence of meiosis is that
only
one
duplication of the chromosomes occurs, but
two
cell divisions occur,
producing four haploid gametes from the original diploid cell. Meiosis requires
