The Cell Cycle, Checkpoints and Cancer

The cycle and its checkpoints

Interphase ($\text{G}_1$, S, $\text{G}_2$) then mitosis. DNA is replicated in S phase only.

Three checkpoints stop the cycle if something is wrong:

• $\text{G}_1$ checkpoint — is the cell big enough, is the DNA undamaged, are growth signals present? This is the main decision point.
• $\text{G}_2$ checkpoint — was DNA replicated completely and correctly?
• M checkpoint — are all chromosomes attached to spindle fibres?

Cancer as checkpoint failure

Two gene classes matter, and they fail in opposite ways:

• Proto-oncogenes normally promote division. A gain-of-function mutation makes an oncogene — the accelerator stuck down. One mutated copy is enough.
• Tumour suppressors such as p53 normally halt division to allow repair. A loss-of-function mutation removes the brake. Usually both copies must fail.