Essential Cell Biology 5th edition

646 CHAPTER 18 The Cell-Division Cycle
(A)
(B)
Figure 18−45 Mutation of the myostatin gene leads to a dramatic increase in muscle mass. (A) This Belgian
Blue was produced by cattle breeders and was only later found to have a mutation in the myostatin gene. (B) Mice
purposely made deficient in the same gene also have remarkably big muscles. A normal mouse is shown at the top
for comparison with the muscular mutant shown at the bottom. (A, Yann Arthus-Bertrand/Getty Images; B, from
S.-J. Lee, PLoS One 2:e789, 2007.) ECB5 e18.45/18.45
ESSENTIAL CONCEPTS
• The eukaryotic cell cycle consists of several distinct phases. In interphase,
the cell grows and the nuclear DNA is replicated; in M phase,
the nucleus divides (mitosis) followed by the cytoplasm (cytokinesis).
• In most cells, interphase consists of an S phase when DNA is duplicated
plus two gap phases: G 1 and G 2 . These gap phases give
proliferating cells more time to grow and prepare for S phase and
M phase.
• The cell-cycle control system coordinates events of the cell cycle by
sequentially and cyclically switching on and off the appropriate parts
of the cell-cycle machinery.
• The cell-cycle control system depends on cyclin-dependent protein
kinases (Cdks), which are cyclically activated by the binding of cyclin
proteins and by phosphorylation and dephosphorylation; when activated,
Cdks phosphorylate key proteins in the cell.
• Different cyclin–Cdk complexes trigger different steps of the cell
cycle: G 1 -Cdk drives the cell through G 1 ; G 1 /S-Cdk and S-Cdk drive it
into S phase; and M-Cdk drives it into mitosis.
• The control system also uses protein complexes, such as APC/C, to
trigger the destruction of specific cell-cycle regulators at particular
stages of the cycle.
• The cell-cycle control system can halt the cycle at specific transition
points to ensure that intracellular and extracellular conditions are
favorable and that each step is completed before the next is started.
Some of these control mechanisms rely on Cdk inhibitors that block
the activity of one or more cyclin–Cdk complexes.
• S-Cdk initiates DNA replication during S phase and helps ensure that
the genome is copied only once. The cell-cycle control system can
delay cell-cycle progression during G 1 or S phase to prevent cells
from replicating damaged DNA. It can also delay the start of M phase
to ensure that DNA replication is complete.
• Centrosomes duplicate during S phase and separate during G 2 . Some
of the microtubules that grow out of the duplicated centrosomes
interact to form the mitotic spindle.