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U N I T 1
Cell and Tissue Function
A cell reproduces by performing an orderly sequence of events called the cell cycle.
The cell cycle is divided into four phases of unequal duration that include the (1)
synthesis (S) and mitosis (M) phases that are separated by (2) two gaps (G
1
and G
2
).
There is also (3) a dormant phase (G
0
) during which the cell may leave the cell cycle.
Movement through each of these phases is mediated at (4) specific checkpoints that
are controlled by specific enzymes and proteins called cyclins.
U N D E R S T A N D I N G
The Cell Cycle
M
G
1
G
2
S
G
0
Synthesis and Mitosis.
Synthesis (S) and mitosis (M) rep-
resent the two major phases of
the cell cycle. The S phase, which
takes about 10 to 12 hours, is the
period of DNA synthesis and repli-
cation of the chromosomes. The M
phase, which usually takes less than
an hour, involves formation of the
mitotic spindle and cell division with
formation of two daughter cells.
1
M
G
1
G
2
S
G
0
Gaps 1 and 2.
Because most cells
require time to grow and double
their mass of proteins and organ-
elles, extra gaps (G) are inserted into
the cell cycle. G
1
is the stage during
which the cell is starting to prepare
for DNA replication and mitosis
through protein synthesis and an
increase in organelle and cytoskel-
etal elements. G
2
is the premitotic
phase. During this phase, enzymes
and other proteins needed for cell
division are synthesized and moved
to their proper sites.
2
