A cell that divides at the wrong moment, or with damaged DNA, can harm the whole body. So the cell cycle has brakes and inspection points, and their failure is what cancer is.
Key idea
Proteins at checkpoints let the cycle continue only when the cell is ready and its DNA is intact. When those controls fail, cells can divide without limit, which is what happens in cancer.
| Checkpoint | When | What it checks |
|---|---|---|
| G1 checkpoint | End of G1 | Is the cell big enough? Are there enough nutrients and growth signals? Is the DNA undamaged? If not, the cell waits or enters G0. |
| G2 checkpoint | End of G2 | Has all the DNA been copied, and copied correctly? |
| M (spindle) checkpoint | Metaphase | Is every chromosome attached to spindle fibers from both poles? |
Why it works
Two kinds of genes do the steering, like the gas pedal and brakes of a car. Proto-oncogenes make proteins that push the cycle forward when the body signals for growth. A mutation can turn one into an oncogene, a gas pedal stuck down. Tumor suppressor genes, such as the gene for the protein p53, make brakes that stop the cycle or tell a badly damaged cell to destroy itself. A mutation can break a brake. Usually several such mutations must pile up in one cell before it divides out of control, which is one reason most cancers become more common with age.
Worked example: Pedal or brake?
A mutation makes a growth-signal protein stay switched on even when no signal arrives. Is the gene now acting like a stuck gas pedal or a broken brake?
- The normal protein pushes the cycle forward only when signaled: a proto-oncogene.
- Now it pushes all the time.
- That is a gas pedal stuck down: an oncogene.
Answer: A stuck gas pedal (an oncogene)
Worked example: Which checkpoint?
A cell has copied its DNA, but some stretches were copied with errors. Which checkpoint should stop it?
- DNA copying happens in S.
- The checkpoint after S and G2, before mitosis, checks that copying is complete and correct.
- So the G2 checkpoint.
Answer: The G2 checkpoint
Common mistake
Thinking cancer cells are invaders from outside the body. They are the body's own cells whose controls on division have broken, usually through several mutations in genes that act as gas pedals and brakes. (Some viruses can raise the risk of certain cancers, but the cancer cells themselves are still the person's own cells.)