QUESTION 71 (5 marks)
An mRNA normally contains the codon GAA. Three independent base substitutions produce GAG, GUA and UAA. Use the code excerpt.
a) Classify each substitution as silent, missense or nonsense. [3 marks]
b) Explain why a missense mutation may have a different effect from a nonsense mutation. [2 marks]
Practice marking scheme
Answer
a) GAG: silent; GUA: missense; UAA: nonsense. b) Missense substitutes one amino acid; nonsense can truncate the chain by introducing a stop.
Working
GAA and GAG both specify glutamate, so the first change does not alter that amino acid. GUA specifies valine, changing one amino acid. UAA is a stop codon and may terminate translation prematurely. The effect of replacing an amino acid depends on its location and role; a premature stop may remove many downstream residues. Neither change has a universally fixed severity.
Marking criteria
- Classifies GAG as silent. [1 mark]
- Classifies GUA as missense. [1 mark]
- Classifies UAA as nonsense. [1 mark]
- Explains replacement of an amino acid by missense. [1 mark]
- Explains possible early termination/truncation by nonsense. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.