In Mutations you met mutations as changes in DNA. With the codon table and translation in hand, you can predict exactly what a point mutation does to a protein.
| Kind | What changes | Example (mRNA) |
|---|---|---|
| Silent | A base changes but the codon still means the same amino acid | GGA → GGG: Gly → Gly |
| Missense | A base change gives a different amino acid | GAG → GUG: Glu → Val |
| Nonsense | A base change turns a codon into a stop codon | UGG → UGA: Trp → stop |
| Frameshift | An insertion or deletion of bases (not a multiple of 3) shifts every codon after it | AUG UUC GGA → AUG UCG GA… |
Key idea
Read the effect from the codon table: silent (same amino acid), missense (different amino acid), nonsense (early stop) or frameshift (reading frame shifted). What the change does to the organism depends on where it falls in the protein and on the environment.
Why it works
Why are some mutations harmless? The code is redundant: most amino acids have several codons, often differing only in the third base, so many third-base changes are silent. Missense changes matter most when they hit an active site or swap R-group families (Amino Acid R Groups). Frameshifts and early stops usually wreck the protein. Mutations are also the raw material of variation: without them, natural selection would have nothing new to act on.
Worked examples
Worked example: Classify the change
An mRNA reads AUG UGG CCA UAA. A mutation changes the G at position 6 to A. What kind of mutation is it?
- The second codon UGG becomes UGA.
- UGG means Trp; UGA is a stop codon.
- The protein ends after Met.
Answer: A nonsense mutation.
Worked example: The environment matters
Sickle-cell hemoglobin comes from one missense change (Glu → Val). Two copies of the allele cause sickle cell disease. Why is the allele more common in regions where malaria is common?
- People with one copy of the allele are resistant to malaria.
- Where malaria is common, that is a survival advantage, so the allele is passed on more often.
- Where malaria is absent, there is no such advantage.
Answer: The same allele helps carriers in one environment and not in another: its effect on fitness depends on the environment.
Common mistake
Assuming a mutation has the same effect in every environment. Whether a change helps, harms or does nothing depends on conditions: the sickle-cell allele is the classic case. Also check frameshifts carefully: deleting three bases removes one codon and keeps the frame; deleting one or two shifts everything after it.