Cell Cycle
Normal cells grow and divide in an orderly fashion, in accordance with
the cell cycle. (Mutations in proto-oncogenes or in tumor suppressor
genes allow a cancerous cell to grow and divide without the normal
controls imposed by the cell cycle.) The major events in the cell cycle
are described in Fig. 2.
Several proteins control the timing of the events in the cell cycle, which
is tightly regulated to ensure that cells divide only when necessary. The
loss of this regulation is the hallmark of cancer. Major control switches
of the cell cycle are cyclin-dependent kinases. Each cyclin-
dependent kinase forms a complex with a particular cyclin, a protein
that binds and activates the cyclin-dependent kinase. The kinase part
of the complex is an enzyme that adds a phosphate to various proteins
required for progression of a cell through the cycle. These added
phosphates alter the structure of the protein and can activate or
inactivate the protein, depending on its function. There are specific
cyclin-dependent kinase/cyclin complexes at the entry points into the
G1, S, and M phases of the cell cycle, as well as additional factors that
help prepare the cell to enter S phase and M phase.
Cell Biology and Cancer 6
Figure 2. The cell cycle is an ordered
process of events that occurs in four
stages. During the two gap phases, G1
and G2, the cell is actively metabolizing
but not dividing. In S (synthesis) phase,
the chromosomes duplicate as a result
of DNA replication. During the M
(mitosis) phase, the chromosomes
separate in the nucleus and the division
of the cytoplasm (cytokinesis) occurs.
There are checkpoints in the cycle at
the end of G1 and G2 that can prevent
the cell form entering the S or M
phases of the cycle. Cells that are not in
the process of dividing are in the G0
stage, which includes most adult cells.