The Biology of Cancer - Vibrational Spectroscopy for Tissue Analysis - page 44

the cell cycle, corresponding to the separation of daughter chromosomes and
usually ending with cell division ( cytokinesis ). However, mitosis and cytoki-
nesis last only about an hour, so approximately 95% of the cell cycle is spent
in interphase or the period between mitosis. During interphase, the chromo-
somes are decondensed and distributed throughout the nucleus and appear
as a network of long, thin threads called chromatin , so the nucleus appears
morphologically uniform. At the molecular level, however, is where both cell
growth and DNA replication occur in an orderly manner in preparation for
cell division.
The cell grows at a steady rate throughout interphase. At some point
before prophase begins, the chromosomes begin to replicate themselves
to form pairs of identical chromosomes, and most dividing cells double in
size between one mitosis and the next. In contrast, DNA is synthesized dur-
ing only a portion of interphase. The timing of DNA synthesis thus divides
the cycle of eukaryotic cells into four discrete phases. The M phase of the
cycle corresponds to mitosis, which is usually followed by cytokinesis. This
phase is followed by the G 1 phase (gap 1), which corresponds to the interval
(gap) between mitosis and initiation of DNA replication. A rapidly dividing
human cell, which divides every 24 hours, spends 9 hours in the G 1 phase [4].
The metabolic rate of the cell will be high. However, a cell may pause in the
G1 phase before entering the S phase and enter a state of dormancy called
the G0 phase. Most mammalian cells do this. G1 consists of four subphases:
1. Competence (G1a)
2. Entry (G1b)
3. Progression (G1c)
4. Assembly (G1d)
During G 1 , the cell is metabolically active and continuously grows but does
not replicate its DNA. G 1 is followed by the S phase (synthesis), during which
DNA replication takes place. At the beginning of the S phase, each chromo-
some is composed of one coiled DNA double helix molecule, which is called
a chromatid . At the end of this stage, each chromosome has two identical DNA
double helix molecules, and therefore is composed of two sister chromatids.
During the S phase, the centrosome is also duplicated. These two events
are unconnected, but require many of the same factors to progress. The end
result is the existence of duplicated genetic material in the cell, which will
eventually be divided into two. Damage to DNA often takes place during
this phase, and DNA repair is initiated following the completion of replica-
tion. The completion of DNA synthesis is followed by the G2 phase (gap 2),
during which cell growth continues and proteins are synthesized in prepa-
ration for mitosis. Although chromosomes have been replicated, they can-
not yet be distinguished individually because they are still in the form of
loosely packed chromatin fibers. This phase is the third, final, and usually the