The big idea: A mutation is a random change in the DNA base sequence. What happens next depends on which cell it occurs in.
A germline mutation happens in a cell that makes gametes (egg or sperm), so it can be inherited — passed on to offspring.
A somatic mutation happens in any other body cell, so it cannot be inherited — it stays in that one individual and only affects the cells that descend from the mutated cell.
- Mutation
- A random change in the base (DNA) sequence of a gene or chromosome.
- Germline mutation
- A mutation in a gamete (egg or sperm) or in a cell that forms gametes. It can be passed on to offspring, so it is heritable.
- Somatic mutation
- A mutation in any body cell other than a gamete-forming cell. It is not passed on to offspring (it is non-heritable).
- Gamete
- A reproductive cell — an egg or a sperm — that joins with another to form offspring.
- Heritable
- Able to be passed from parent to offspring through the gametes.
| Feature | Germline mutation | Somatic mutation |
|---|---|---|
| Which cell it happens in | A gamete-forming cell (in the ovary or testis), or a gamete (egg or sperm) | Any other body cell (skin, lung, gut, etc.) |
| Can it be inherited? | YES — passed to offspring through the egg or sperm | NO — not passed to offspring; it stays in that individual |
| Who is affected | Every cell of the offspring carries it | Only the cells descended from the one mutated cell |
| Example | A mutation in a sperm cell that a child then inherits | A mutation in a lung cell caused by tobacco smoke |
The one question that decides it: To tell germline from somatic, ask: could this mutation be passed to a child?
If the mutated cell is a gamete (egg/sperm) or makes gametes → germline → heritable.
If it is any other body cell (skin, lung, gut...) → somatic → not heritable.
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Mutations happen at a low rate all the time. But some agents make them much more likely — these are called mutagens.
When a mutagen damages a body cell's DNA and the mutation lands in a gene that controls cell division, the result can be cancer.
- Mutagen
- An agent that increases the rate of mutation — for example ultraviolet (UV) light, X-rays, or chemicals in tobacco smoke.
- Carcinogen
- A mutagen that specifically increases the risk of cancer (for example the chemicals in tobacco smoke).
- Tumour
- A mass of cells produced by uncontrolled cell division.
- Cancer
- A disease in which body cells divide uncontrollably, forming a tumour that can invade and spread to other tissues.
- Metastasis
- The spread of cancer cells from the original tumour to other parts of the body.
Mutagens — what raises the mutation rate: A mutagen is anything that makes mutations more likely. Common examples:
Radiation — ultraviolet (UV) light, X-rays and gamma rays.
Chemicals — the many chemicals in tobacco smoke, and some industrial chemicals.
A mutagen that causes cancer is also called a carcinogen.
| Mutagen | Type | Everyday example |
|---|---|---|
| Ultraviolet (UV) light | Radiation | Strong sunlight / sunbeds → skin-cell mutations |
| X-rays / gamma rays | Radiation | High doses of ionising radiation |
| Chemicals in tobacco smoke | Chemical | Smoking → lung-cell mutations |
| Some industrial chemicals | Chemical | Certain solvents and pollutants |
How a mutation becomes cancer: Cancer develops as a chain of events:
A mutagen damages a cell's DNA → a mutation arises in a gene that controls cell division → the cell divides uncontrollably → over time several mutations accumulate in that cell line → a tumour forms, which may become malignant and spread (metastasis).
Cancer usually needs an accumulation of several mutations, not just one — which is why the risk rises with repeated exposure and with age.
| Step | What happens | Why it matters |
|---|---|---|
| 1. Mutagen acts | A mutagen (e.g. tobacco chemicals, UV light) damages the DNA of a body cell | Mutagens raise the mutation rate |
| 2. Mutation occurs | A mutation arises in a gene that controls cell division / the cell cycle | The cell loses its normal 'stop' signals |
| 3. Uncontrolled division | The cell divides repeatedly and uncontrollably | Cells are made faster than the body needs |
| 4. Mutations accumulate | Several mutations build up in the same cell line over time | More controls are lost; the cell becomes more abnormal |
| 5. Tumour forms | The mass of dividing cells forms a tumour (which may become malignant and spread) | A malignant tumour that spreads is cancer |
Germline mutation
- Happens in a gamete (egg/sperm) or gamete-forming cell
- Can be inherited by offspring
- Found in every cell of any child who inherits it
- Does not cause cancer in the parent
Somatic mutation
- Happens in any other body cell (skin, lung, gut...)
- Cannot be inherited
- Found only in cells descended from the mutated cell
- Can cause cancer if it hits a cell-division gene
A memory hook: Mutagen → mutation → uncontrolled division → tumour. That is the whole cancer story in four steps.
And cancer is almost always a somatic event: it arises in body cells, so it is not inherited — only the risk (from inherited genes) can run in families.
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How this is tested: A 1-mark Identify question often gives a list of cells and asks which one's mutation could be inherited — the answer is a gamete-forming (germline) cell, such as a cell in the testis or ovary, or a sperm or egg.
A short Explain item can ask for the mechanism by which smoking causes cancer — link the chemicals (mutagens) to mutations in lung-cell DNA, then to uncontrolled division.
The bigger 4-mark Outline asks you to outline how a mutation can lead to cancer — give the separate scoring steps from mutagen through to tumour.
IB-style question — outline how mutation leads to cancer
Outline how a mutation can lead to the development of cancer. [4]
How to score all four marks
- Start with the cause. A mutagen (such as the chemicals in tobacco smoke or UV light) causes a mutation in the DNA of a body cell.
- Hit a control gene. The mutation occurs in a gene that controls cell division (the cell cycle), so the cell's normal 'stop' signals are lost.
- Uncontrolled division. The affected cell then divides uncontrollably, producing far more cells than the body needs.
- Accumulate and form a tumour. As several mutations accumulate in that cell line, the dividing cells form a tumour, which may become malignant and spread. (Award 1 mark per distinct point, up to 4.)
Final answer
A mutagen causes a mutation in a gene controlling cell division; the cell then divides uncontrollably; further mutations accumulate; and the dividing cells form a tumour that can become malignant.
Why this scores full marks: Each sentence is a separate, distinct step — mutagen → mutation in a cell-division gene → uncontrolled division → accumulation → tumour.
A 4-mark 'outline' needs four scoring points in a clear cause-and-effect chain, not one idea written four ways.
| Step | What happens | Why it matters |
|---|---|---|
| 1. Mutagen acts | A mutagen (e.g. tobacco chemicals, UV light) damages the DNA of a body cell | Mutagens raise the mutation rate |
| 2. Mutation occurs | A mutation arises in a gene that controls cell division / the cell cycle | The cell loses its normal 'stop' signals |
| 3. Uncontrolled division | The cell divides repeatedly and uncontrollably | Cells are made faster than the body needs |
| 4. Mutations accumulate | Several mutations build up in the same cell line over time | More controls are lost; the cell becomes more abnormal |
| 5. Tumour forms | The mass of dividing cells forms a tumour (which may become malignant and spread) | A malignant tumour that spreads is cancer |