The big idea: Cell division is controlled by genes. Mutations that turn proto-oncogenes into oncogenes, or that disable tumour suppressor genes, can lead to uncontrolled cell division.
The points to remember
- Proto-oncogenes code for proteins that stimulate the cell cycle when division is needed.
- A mutation can turn a proto-oncogene into an oncogene, which stimulates division all the time.
- Tumour suppressor genes code for proteins that slow or stop the cell cycle, for example when DNA is damaged.
- A mutation can stop a tumour suppressor gene working, so division is no longer held back.
- Uncontrolled division forms a tumour; this can lead to cancer.
Remember it as: Accelerator stuck on, brake cut: no control.
Real example: TP53. The tumour suppressor gene TP53 codes for the protein p53, which can stop the cycle when DNA is damaged. TP53 is mutated in about half of all human cancers.
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How mutations add up: One mutation is rarely enough. Cancer usually needs several mutations in genes that control the cell cycle, all in the same line of cells.
| Gene | Normal role | After mutation |
|---|---|---|
| Proto-oncogene | Stimulates division when needed | Oncogene: stimulates division all the time |
| Tumour suppressor gene | Slows or stops division | Not working: division not held back |
Mutagens raise the risk: Mutagens, such as chemicals in tobacco smoke and UV radiation, increase the chance of these mutations. Mutations also build up over a lifetime, which is why cancer is more common in older people.
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How it is asked: A short Paper 2 'explain how chemicals (or radiation) can cause cancer', or 1A items on what oncogenes and tumour suppressor genes do.
IB-style questionExplain[2 marks]
Explain how a chemical in the diet could cause cancer.
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