The big idea: Gene mutations are caused by mutagens, which damage DNA, and by errors when DNA is replicated or repaired.
The points to remember
- A mutagen is anything that increases the rate of mutation.
- Chemical mutagens: for example chemicals in tobacco smoke, which attach to bases.
- Mutagenic radiation: UV light, X-rays and gamma rays, including radiation from radioactive isotopes.
- Replication errors: a wrong base added and not corrected by proofreading.
- Repair errors: damaged DNA restored with the wrong base sequence.
Remember it as: Chemicals, radiation, copying slips, repair slips.
Real example: tobacco smoke. Tobacco smoke contains benzo[a]pyrene, a chemical mutagen. It attaches to guanine bases in the DNA of lung cells, so wrong bases are put in when the DNA is copied.
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Mutagens and mistakes: Mutagens raise the mutation rate; copying and repair errors happen even without them.
| Cause | Examples | What it does to DNA |
|---|---|---|
| Chemical mutagens | Benzo[a]pyrene in tobacco smoke, some food toxins | Attach to or change bases, so they pair wrongly |
| UV radiation | Sunlight, sunbeds | Joins neighbouring bases (two thymines) |
| High-energy radiation | X-rays, gamma rays, radioactive isotopes such as radon | Breaks DNA strands; bases lost or changed |
| Replication errors | DNA polymerase adds a wrong base | A mismatch not fixed by proofreading |
| Repair errors | Faulty repair of damaged DNA | The wrong base sequence put back |
Mutagen, not 'random': Name the cause: a mutagen (chemical or radiation) or an error in replication. 'It happened randomly' does not name a cause.
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How it is asked: Naming a type of mutation a mutagen causes, listing ways mutations arise, or describing how a named mutagen such as UV or a radioactive isotope changes DNA.
IB-style questionOutline[3 marks]
Outline three different causes of gene mutation.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.