Biomedical Engineering Reference
In-Depth Information
In the
Gap-1
(G-1) period the DNA of the chromosome is not yet
duplicated and each chromosome consists of a single double helix.
At this period, the enzyme systems needed to duplicate this DNA
are mobilized. This period ends when DNA duplication starts.
The
Synthetic
(S) period starts with the DNA replicated at different
points along the double helix at sites called replicons. Each chro-
mosome has numerous replicating sites, which have different start-
ing points in the S period. Some chromosomes replicate their DNA
early in the S phase, while others, such as the inactive X chromo-
some of females, replicate their genetic material late in this period.
Replication of the DNA occurs in a semi-conservative manner, in
which the old strands unwind and are each copied, so the new double
helix is composed of one old and one new strand. At the end of the
S phase each of the chromosomes has duplicated its DNA and has
two chromatids. Each chromatid is composed of one DNA double
helix; these chromatids will become the Gap-1 chromosomes in the
daughter cells. This period, which lasts from the time of completion
of DNA synthesis to the start of mitosis, is called the
Gap-2
(G-2)
period of interphase.
The 'active' stage of the cell cycle is
mitosis
, since this is the period of active
chromosome movement, which can be followed under the light microscope.
Mitosis is classically broken up into four stages: (1)
Prophase
begins with
the condensation of the chromosomes so they become visible using a light
microscope. At this stage each chromosome is composed of two chroma-
tids connected at a primary constriction called the centromere. The nuclear
membrane disappears at this point. (2) At
metaphase
the chromosomes
move to a central plane within the cell called the metaphase plate. Spindle
fi bers connect to the kinetochore of the centromere of each chromosome.
The spindle fi bers are connected to the centrioles, located at opposite poles
of the cell. (3) In
anaphase
the centromeres of each of the chromosomes
divide and the chromatids are pulled to the poles of the cell by contraction
of the spindle fi bers. (4)
Telophase
is the fi nal stage of mitosis, where the
chromatids reach the poles of the cell and start to de-condense to form the
chromatin fi bers of the new interphase cell. The nuclear membrane starts to
reform and the cytoplasm divides. The two new daughter cells are geneti-
cally identical to the original cell (Fig. 7.10).
Mutations to the DNA or the separation of the chromosomes and their
distribution to daughter cells can occur during this mitotic process, yielding
a clone of cells that have altered genetic information. Thus, each division is
accompanied with numerous cellular checkpoints and proofreading mecha-
nisms to insure fi delity to the original genetic information. Still, even with
these meticulous safeguards, mutations to somatic cells occur and can lead
