← Gene expression
Mutation types, mildest to most severe
Every point mutation is a substitution — the severity question is really about what happens to the amino acid it produces.| Type | What happens to the codon | Effect on protein |
|---|---|---|
| Silent / same-sense | Codon changes, but still codes the SAME amino acid (degeneracy) | No effect — protein unchanged |
| Missense | Codon changes to a DIFFERENT amino acid | May or may not change function, depending on the amino acid and its role |
| Nonsense | Codon changes to a STOP codon | Protein cut short — usually non-functional |
| Frameshift (insertion/deletion) | Every codon downstream shifts | Usually wrecks the whole protein from that point on |
Substitutions can only ever be silent, missense, or nonsense — a frameshift is never caused by a substitution, only by an insertion or deletion.
ℹWhy two separate steps, not one
DNA stays in the nucleus, protected, and can be transcribed into many mRNA copies for repeated use — while ribosomes in the cytoplasm can only read single-stranded mRNA, not double-stranded DNA. Two steps also allow the cell to control gene expression at the transcription stage, before any protein is made.Silent vs missense vs nonsense — the quick test
- Same amino acid → SILENT.
- Different amino acid, protein may still work → MISSENSE.
- A STOP codon appears early → NONSENSE.
Practice for this standard covers the whole topic — flashcards and quiz.