Mutations: When the Copy Is Wrong
One base in three billion. Sometimes it means nothing at all, sometimes it means sickle-cell anaemia — and the difference is mostly about reading frames.
Builds on: 2.3 The Genetic Code: From Gene to Protein2.2 Replication: Copying Three Billion Letters
A change in the sequence
A mutation is any change to the DNA sequence. Most arise as uncorrected copying errors; others are caused by mutagens — UV light, X-rays, tobacco smoke, certain chemicals. They occur constantly, and the vast majority are harmless because they land in non-coding DNA or get repaired.
The ones inside a gene fall into two very different classes.
Substitutions: one letter swapped
One base is replaced by another. Three outcomes are possible:
- Silent — the new codon still means the same amino acid. GGA → GGG is still glycine. Thank wobble; these do nothing at all.
- Missense — a different amino acid is inserted. The effect depends entirely on which one and where. Often harmless; occasionally decisive.
- Nonsense — the codon becomes a STOP. The protein is cut short and almost always useless.
The famous missense case is sickle-cell anaemia. One base changes GAG to GTG in the haemoglobin gene, swapping glutamate for valine — a single amino acid out of 146. Glutamate is charged and water-friendly; valine is oily. That one substitution makes haemoglobin molecules stick together into fibres, deforming red blood cells into crescents that jam capillaries. One letter in three billion, and a serious disease.
The same mutation, in a single copy, also confers resistance to malaria — which is why it stayed common in populations where malaria was endemic. Whether a mutation is “bad” depends on the environment, which is precisely the point of Unit 4.
Frameshifts: everything after it
Insert or delete a base and you do not change one codon — you change the grouping of every codon downstream. The reading frame slips by one, and everything after the mutation is read as a different set of triplets entirely.
The sentence does not lose a word; it loses all meaning from that point on. A frameshift usually produces a garbled protein and hits a premature STOP somewhere in the nonsense. This is why insertions and deletions are, on average, far more damaging than substitutions — and why a deletion of exactly three bases is much milder, since it removes one amino acid and leaves the frame intact.
Where the mutation happens matters
- Somatic — in a body cell. Affects you, is not inherited. Cancer is what happens when somatic mutations accumulate in genes controlling cell division.
- Germline — in eggs or sperm. Does not affect you, but is passed to every cell of your children. Only germline mutations feed evolution.
It is tempting to treat mutation as purely damage, but every allele that ever existed — every eye colour, every enzyme variant, every adaptation in the history of life — began as a copying error. Selection can only choose among variants that already exist, and mutation is the only thing that makes new ones.
⚗️ Lab — The Mutation Bench
One gene, before and after. Pick a mutation and see what reaches the protein.
- Substitute a third base in a codon. Often the protein is unchanged — a silent mutation.
- Now delete a single base. Compare how much of the protein survives.
- Find a substitution that creates an early STOP. That is a nonsense mutation.