Count in threes. A swap rewrites one codon. A single added or lost base rewrites every codon after it.
A mutation is a random change in DNA. In a coding sequence, its effect depends on what it does to the codons: nothing, one swapped amino acid, an early stop, or a scrambled reading frame.
Key terms
- Substitution — one base swapped for another. Only one codon changes, and the reading frame stays the same.
- Silent mutation — the new codon codes for the same amino acid. The DNA changed; the protein did not.
- Missense mutation — the new codon codes for a different amino acid. The effect ranges from harmless to severe.
- Nonsense mutation — the new codon is a stop codon. Translation ends early, and the protein comes out short.
- Frameshift — an insertion or deletion that isn't a multiple of three bases. Every later codon is misread.
- Redundancy — 61 codons name only 20 amino acids. Most amino acids have several codons, usually differing only in the third base.
How it works
- Mutations come from copying errors and from damage by UV, other radiation, or reactive chemicals. They arise at random, not because the organism needs them.
- Most copying errors get fixed. DNA polymerase proofreads as it copies, and mismatch repair checks behind it. Together they leave about one error per billion bases.
- A change in coding DNA is copied into mRNA. The ribosome reads mRNA in threes from the start codon, which sets the frame.
- A substitution rewrites one codon → silent, missense, or nonsense. An insertion or deletion not in threes shifts the frame → every codon after it is misread.
- Mutations in regulators spread. Alter a transcription factor, and expression of every gene it controls can change at once.
- Whether a protein change helps or hurts can depend on the environment. The same change can be harmful in one setting and useful in another.
Why it matters
Mutation is the ultimate source of new alleles, so it feeds Unit 7: selection can only sort variants that already exist. Errors can also hit whole chromosomes: nondisjunction in meiosis can leave a gamete with an extra or missing chromosome, which is how trisomy 21 usually arises. On the exam, find what happened to the codon first, then predict the protein.
Exam traps
| Trap | Actually |
|---|
| Every mutation changes the protein. | Many substitutions, especially at the third base, are silent. The DNA changes; the amino acids don't. |
| A short protein means a frameshift. | A nonsense mutation also makes one, ending at the new stop codon. A frameshift ends wherever the shifted frame hits a stop. |
| Any deletion scrambles the rest of the protein. | Delete three bases and the frame holds. The protein comes out one amino acid shorter, and the rest reads normally. |
| A mutant transcription factor changes the amino acid sequence of its target proteins. | It changes how much of each target gets made. The target genes, and their sequences, are untouched. |
Worked example
A bacterial enzyme is 300 amino acids long. Strain A has one base deleted in codon 20 and makes no working enzyme. Strain B has the same deletion plus one extra base inserted in codon 23, and its enzyme works almost normally. Explain why the second mutation restores function.
- Losing one base in codon 20 shifts the frame. Every codon after it is misread, so strain A's enzyme is wrong from codon 20 on.
- Adding one base in codon 23 shifts the frame back: −1 + 1 = 0. From codon 24 on, the ribosome reads the original codons again.
- Only codons 20 to 23 are misread, so about four amino acids change. If none of them is in the active site, and none became a stop codon, the enzyme still folds and works.
The insertion restores the reading frame three codons after the deletion. Only about four amino acids near codon 20 are wrong; the other 296 or so are normal, so the enzyme mostly works.
A frameshift depends on the net number of bases gained or lost. Here that is −1 + 1 = 0, so the frame comes back. The near miss is 'the insertion put the missing base back.' It didn't: it added a base in a different codon. What came back was the frame, not the original sequence.