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.
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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.