Essential Cell Biology 5th edition

Overview of the Cell Cycle
611
TABLE 18−1 SOME EUKARYOTIC CELL-CYCLE DURATIONS
Cell Type
Early fly embryo cells
Early frog embryo cells
Mammalian intestinal epithelial cells
Mammalian fibroblasts in culture
Duration of Cell Cycle
8 minutes
30 minutes
~12 hours
~20 hours
The Eukaryotic Cell Cycle Usually Includes Four Phases
Seen in a microscope, the two most dramatic events in the cell cycle are
when the nucleus divides, a process called mitosis, and when the cell
itself then splits in two, a process called cytokinesis. These two processes
together constitute the M phase of the cycle. In a typical mammalian cell,
the whole of M phase takes about an hour, which is only a small fraction
of the total cell-cycle time (see Table 18−1).
The period between one M phase and the next is called interphase.
Viewed with a microscope, it appears, deceptively, as an uneventful
interlude during which the cell simply increases in size. Interphase, however,
is a very busy time for a proliferating cell, and it encompasses the
remaining three phases of the cell cycle. During S phase (S = synthesis),
the cell replicates its DNA. S phase is flanked by two “gap” phases—called
G 1 phase and G 2 phase—during which the cell continues to grow (Figure
18−2). During these gap phases, the cell monitors both its internal state
and external environment. This monitoring ensures that conditions are
suitable for reproduction and that preparations are complete before the
cell commits to the major upheavals of S phase (which follows G 1 ) and
mitosis (following G 2 ). At particular points in G 1 and G 2 , the cell decides
whether to proceed to the next phase or pause to allow more time to
prepare.
During all of interphase, a cell generally continues to transcribe genes,
synthesize proteins, and grow in mass. Together with S phase, G 1 and
G 2 provide the time needed for the cell to enlarge and to duplicate its
cytoplasmic organelles. If interphase lasted only long enough for DNA
replication, the cell would not have time to double its mass before it
divided and would consequently shrink with each division. Indeed, in
some special circumstances that is exactly what happens. In an early frog
embryo, for example, the first cell divisions after fertilization (called cleavage
divisions) serve to subdivide the giant egg cell into many smaller cells
M PHASE
mitosis
(nuclear
division) cytokinesis
(cytoplasmic
G 2 PHASE
M division)
G 2
INTERPHASE
S
G 1
S PHASE
(DNA replication)
G 1 PHASE
Figure 18–2 The eukaryotic cell cycle
usually occurs in four phases. The cell
grows continuously during interphase, which
consists of three phases: G 1 , S, and G 2 . DNA
replication is confined to S phase. G 1 is the
gap between M phase and S phase, and
G 2 is the gap between S phase and
M phase. During M phase, the nucleus
divides in a process called mitosis; then
the cytoplasm divides, in a process
called cytokinesis. In this figure—and in
subsequent figures in the chapter—the
lengths of the various phases are not drawn
to scale: M phase, for example, is typically
much shorter and G 1 much longer than
shown.