Cigarette smoking as a behavioural carcinogen
Cigarette smoking is a behavioural factor (a lifestyle choice) that is also a carcinogen (an environmental agent that can cause cancer), because its chemicals damage DNA.
Remember
Smoking is both a behaviour and a carcinogen: its smoke chemicals form DNA adducts (chemical groups stuck on to a DNA base), unrepaired adducts become mutations, and stopping lowers the risk.
A behavioural factor is something a person does, so it can be changed. A carcinogen is an environmental agent that can cause cancer, often by mutating DNA (a mutation is a lasting change in the DNA sequence of a cell). Cigarette smoking sits in both groups at once, and it is the clearest example of a behaviour that is also a carcinogen.
Tobacco smoke is a complex mixture of chemicals, and several of them are genotoxic, meaning that they damage DNA. Many of these form DNA adducts, which are chemical groups stuck on to a DNA base, so the cell misreads the base when it copies its DNA. If an adduct is not repaired first, the misreading becomes a permanent mutation. Mutations of this kind can hit growth-control genes such as p53 (a tumour suppressor gene, which normally puts the brakes on cell division).
Mutations in genes that control the cell cycle matter most. A proto-oncogene (a normal gene that encourages growth) can mutate into an oncogene (a gene stuck in the 'on' position), or a tumour suppressor gene can be lost. Smoking exposes the lungs, and many other tissues, to this mixture again and again, which gives mutations many chances to arise.
Smoking is also the main modifiable risk factor for cancer (modifiable means a person can change it), so stopping lowers the risk. It raises risk rather than guaranteeing cancer in every smoker. Excess weight and inactivity mainly raise risk through hormones, insulin and inflammation, rather than by chemically damaging DNA.
Key facts
- Behavioural factor
- a lifestyle choice, so it can be changed.
- Carcinogen
- an environmental agent that can cause cancer, for example by mutating DNA.
- Smoking mechanism
- chemicals in tobacco smoke form DNA adducts, and unrepaired damage becomes mutation.
- Risk, not certainty
- exposure raises the chance of cancer, but not everyone exposed develops it.
- Not behaviours
- age, sex and inherited gene changes are intrinsic (they belong to the person) or genetic factors.
- Avoidable cancers
- about 4 in 10 cancers in the UK could be prevented by lifestyle changes.
Exam traps
- Obesity is a recognised modifiable cancer risk factor, but altered hormones, insulin and chronic inflammation raise risk indirectly, so they are not a way of chemically damaging DNA as tobacco carcinogens do.
- Cancer-driving mutations activate proto-oncogenes (normal genes that encourage growth), turning them into oncogenes, rather than inactivating them.
- A promoter (an agent that pushes already mutated cells to multiply without itself mutating DNA) is a genuine category of carcinogen, but smoking acts through carcinogens that form DNA adducts and cause mutations, not through growth promotion without DNA damage.
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