Biology (Holt)

Control of the Cell Cycle

If a cell spends 90 percent of its time in interphase, how do cells

“know” when to divide? How is the cycle controlled? Just as traffic

lights control the flow of traffic, cells have a system that controls

the phases of the cell cycle. Cells have a set of “red light–green light”

switches that are regulated by feedback information from the cell.

The cell cycle has key checkpoints (inspection points) at which

feedback signals from the cell can trigger the next phase of the cell

cycle (green light). Other feedback signals can delay the next phase

to allow for completion of the current phase (yellow or red light).

The cell cycle in eukaryotes is controlled by many proteins. Con-

trol occurs at three principal checkpoints, as shown in Figure 7.

1. Cell growth (G 1 ) checkpoint.This checkpoint makes the deci-
   sion of whether the cell will divide. If conditions are favorable
   for division and the cell is healthy and large enough, certain
   proteins will stimulate the cell to begin the synthesis (S) phase.
   During the S phase, the cell will copy its DNA. If conditions are
   not favorable, cells can typically stop the cell cycle at this check-
   point. The cell cycle will also stop at this checkpoint if the cell
   needs to pass into a resting period. Certain cells, such as some
   nerve and muscle cells, remain in this resting period perma-
   nently and never divide.


2. DNA synthesis (G 2 ) checkpoint. DNA replication is checked at
   this point by DNA repair enzymes. If this checkpoint is passed,
   proteins help to trigger mitosis. The cell begins the many mo-
   lecular processes that are needed to proceed into mitosis.


3. Mitosis checkpoint.This checkpoint triggers the exit from
   mitosis. It signals the beginning of the G 1 phase, the major
   growth period of the cell cycle.

When Control Is Lost: Cancer

Certain genes contain the information nec-

essary to make the proteins that regulate cell

growth and division. If one of these genes is

mutated, the protein may not function, and

regulation of cell growth and division can be

disrupted. ,the uncontrolled growth

of cells, may result. Cancer is essentially a

disorder of cell division. Cancer cells do not

respond normally to the body’s control

mechanisms.

Some mutations cause cancer by over-

producing growth-promoting molecules,

thus speeding up the cell cycle. Others

cause cancer by inactivating the control

proteins that normally act to slow or stop

the cell cycle.

Cancer

126 CHAPTER 6Chromosomes and Cell Reproduction

Reviewing Information

Learn the stages of inter-

phase by reviewing the

steps numbered 1–5 on the

previous page. You can see

in Figures 6 and 7 that the

cell cycle is a repeating

series of three steps

followed by mitosis

and cytokinesis.

G 1

G 1

checkpoint

G 2

checkpoint

Mitosis

checkpoint

G 2

Mitosis

Cytokinesis

S

INTER

PHA

SE

Figure 7 Control of the
cell cycle.The cell cycle in
eukaryotes is controlled at
three inspection points, or
checkpoints. Many proteins
are involved in the control of
the cell cycle.