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CASE FILES: BIOCHEMISTRY
D ISC U SSIO N
The
cell cycle
is defined as the time interval between cell divisions in prolif-
erating cells. It is important to note that the cell cycle is not a simple “clock.”
Movement through the cell cycle is controlled by a variety of proteins that
allow the cell to respond to various stimuli. The eukaryote cell cycle is com-
posed of the following
four phases
(Figure 3-1):
1.
M phase—mitosis
2.
Gj phase (gap 1)—between mitosis and initiation of DNA synthesis
3.
S phase—when DNA synthesis occurs
4.
G2 phase (gap 2)—cell growth and macromolecule synthesis
Although there is great variation in the length of the mammalian cell cycle
(hours to days), as a generalization, mammalian cells divide once every 24 hours.
The M and S phases of the cell cycle are relatively constant. Therefore, the
length of the mammalian cell cycle is determined by length of the G1 and G2
phases.
Cell division
occurs during
M phase, and the mitotic interphase
is
composed of (Gj + S + G2).
M phase, or mitosis,
is divided into four subphases,
prophase, metaphase, anaphase, and telophase.
In
prophase,
the nuclear
membrane breaks down while the replicated chromosomes condense and are
Figure 3-1.
Cell cycle. (Gj = cell growth; S = DNA synthesis and replication;
G2 = proteins made in preparation for cell division; M = mitosis; G0 = rest phase.)
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