Recognising gene mutations
A gene mutation changes the base sequence. You see its effect by comparing the mRNA codon by codon and translating with the genetic code.
Rules
- Silent mutation: new codon, same amino acid
- Missense: a different amino acid
- Nonsense: the codon becomes a stop codon and the protein ends early
- Frameshift: inserting or deleting one base shifts every following codon
Your turn
Three problems on this topic. Type probabilities as a percentage number, e.g. 18.75.
Task
The mRNA of a gene reads 5′-AUG ACG CAU UGG CAC GCU UAA-3′. A mutation turns it into 5′-AUG ACG CAU UAG CAC GCU UAA-3′. What kind of mutation is it, and in which codon (AUG = 1) does it occur?
| Codon (mRNA) | Amino acid |
|---|---|
| ACG | Thr |
| AUG | Met |
| CAC | His |
| CAU | His |
| GCU | Ala |
| UAA | Stop |
| UAG | Stop |
| UGG | Trp |
Type of mutation
Number of the codon affected
Probabilities as a percentage number (25 or 18.75), rounded to 2 decimal places; allele frequencies as a decimal (0.02).
Worked example
The mRNA of a gene reads 5′-AUG CCG GUU GUU UGG UUG UAA-3′. A mutation turns it into 5′-AUG CCG GGU GUU UGG UUG UAA-3′. What kind of mutation is it, and in which codon (AUG = 1) does it occur?
Worked solution
- 1.Compare codon by codon: the first one that differs is no. 3 (GUU → GGU)
- 2.GUU = Val, GGU = Gly
- 3.So: missense mutation (amino acid changed)
Typical mistake: Assuming every base change is harmful. Because the code is degenerate, a change in the third position often changes nothing (silent mutation).
Common questions
Which mutation is the most serious?
Usually frameshifts and nonsense mutations, because the protein looks completely different afterwards or ends early. Silent mutations have no effect.