Biology Reference
In-Depth Information
chromosome condenses to form two visible, thin threads (chromatids) within the
nucleus. Because chromosome duplication occurred in the S phase, each chromo-
some consists of two chromatids connected at the
centromere
. The centromere
is the attachment point for the spindle fibers that will draw each of the chromo-
somes into their respective nuclei later in mitosis. In late prophase, the nuclear
membrane disappears and a mitotic spindle begins to form.
During prometaphase, the spindle develops. The spindle is a complex struc-
ture consisting of centrosomes (two centrioles oriented at right angles to one
another) and microtubules (hollow protein cylinders consisting of tubulin). The
two bundles of fibers extend between the opposite poles of the cell and attach
to the centromere of each chromosome (
Wolf 1995, Gonzalez et al. 1998
). Then,
the chromosomes move toward the center of the cell in a plane equidistant
from the spindle poles. By the end of
metaphase
, the duplicated chromosomes
are lined up on the metaphase plate and are at their most condensed stage,
making it easy to examine them for differences in morphology.
During the next stage,
anaphase
, the centromeres divide and the two sister
chromatids now have their own centromeres, so they have become independent
chromosomes. These newly separated chromosomes move toward the opposite
poles. At the end of anaphase, a complete set of chromosomes lies near each
opposite pole.
During
telophase
, the chromosomes have reached the spindle poles, and the
cleavage furrow within the cytoplasm has become visible. The nuclear mem-
brane reforms around each group of chromosomes, the chromosomes decon-
dense, cleavage progresses, and the spindle disappears. The mitochondria often
align parallel to the spindle, which may guarantee that they are distributed to
both daughter cells. A gradually deepening furrow divides the cytoplasm, and a
new cell membrane forms. If all has gone well, the result should be the forma-
tion of two nearly identical cells with perfectly duplicated genetic information
in the nucleus and in the mitochondria within the cytoplasm.
Check points
occur during the cell cycle to ensure that genetic information is
duplicated perfectly. During the checkpoints, the genetic material is monitored
for integrity and status of replication before the cells commit either to replicate
the DNA during S phase, or to segregate it during mitosis (
Elledge 1996
). If the
cell cycle were not well regulated, the cell would subject to genetic instability or
death.
Cyclins and cyclin-dependent protein kinases regulate the cell cycle (
King
et al. 1996, Stillman 1996, Piwnica-Worms 1999
). The checkpoints involve
