# Point mutation

A change in one DNA base. A silent mutation leaves the amino acid the same. A missense mutation changes it. A nonsense mutation makes a stop codon too early.

Biology (university) · Part 04: DNA, transcription and translation · Card 9 of 9 · Last updated 2026-10-19

## Analogy

Changing one letter in a word: sometimes harmless, sometimes it changes the meaning.

## In practice

Sickle cell disease comes from one base change that swaps one amino acid in haemoglobin.

## Practice

### Flashcards

- **What is a nonsense mutation?** A base change that turns an amino acid codon into a stop codon, so the protein is cut short.

### Interview questions

**Compare silent, missense and nonsense mutations and say which is likely to be most harmful.**

- Silent: same amino acid
- Missense: different amino acid
- Nonsense: premature stop
- Nonsense usually most damaging

## Questions people ask

### Are all mutations harmful?

No. Many are silent and have no effect. Some are harmful, such as the one causing sickle cell disease. A few are helpful and are the raw material of evolution, for example a change that lets adults digest milk sugar. Whether a mutation matters depends on where it happens and the environment.

### What is a frameshift mutation?

A frameshift occurs when bases are added or deleted in a number that is not a multiple of three. The ribosome reads in threes, so every codon after the change is read in the wrong frame. This usually produces a completely different and non-working protein.

## Related concepts

- [Genetic code](https://studyvisually.com/learn/science/biology-university/biology-university-part-4/biology-university-part-4-concept-7)

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