A mutation is a change in the DNA sequence. Copying errors in replication, radiation and some chemicals can cause them. What a mutation does depends on how it changes the codons you learned to read in Reading the Genetic Code.
Key idea
A substitution swaps one base. It can be silent (same amino acid), missense (a different amino acid) or nonsense (a stop codon, so the protein is cut short). An insertion or deletion of one base shifts the reading frame: a frameshift changes every codon after it.
Why it works
The ribosome reads in threes from AUG with no commas. A substitution touches one codon only. An insertion or deletion moves every later base by one, so every later codon is read wrong, the way "THE CAT ATE THE RAT" becomes "THE CAA TET HER AT" if you drop one letter.
| Change to AUG UAC GGC UAA | Effect | Type |
|---|---|---|
| UAC → UAU | Tyr → Tyr | silent |
| UAC → CAC | Tyr → His | missense |
| UAC → UAA | Tyr → stop | nonsense |
| delete the A of UAC: AUG UCG GCU AA… | Tyr → Ser, Gly → Ala, … | frameshift |
Worked example: Name the mutation
The mRNA AUG-CAU-GGA-UGA changes to AUG-CAC-GGA-UGA. What kind of mutation is it?
- Only the second codon changed: CAU → CAC.
- CAU is His. CAC is also His.
Answer: Silent: the protein is unchanged.
Worked example: A famous missense mutation
In sickle cell hemoglobin, one codon of the beta-globin gene changes from GAG to GUG. What changes in the protein?
- GAG codes for glutamic acid (Glu).
- GUG codes for valine (Val).
Answer: One amino acid: Glu becomes Val. That single change alters how the hemoglobin behaves.
Most mutations are neutral or harmful, but a few help, and those are the raw material of natural selection. Only mutations in cells that become gametes are passed to offspring.
Common mistake
It is natural to think every substitution changes the protein. Many do not: most amino acids have several codons, so a change in the third base is often silent. In the other direction, one deleted base can garble the whole rest of a protein. Check by translating both versions, codon by codon.