CAIE A Level Biology 9700 · Nucleic acids and protein synthesis

Nucleic acids and protein synthesis: explain question

Explain6 marksChallenge

A bacterial gene encoding a short polypeptide has the following sequence for its coding (non-template) strand:

5' – ATG GCA TTG TGA – 3'

This sequence is used in transcription to produce mRNA, which is then translated into a polypeptide. Two different mutations occur in separate copies of this gene:

Mutation 1: a substitution that changes the third base of the second codon from A to G.
Mutation 2: a deletion of the first base of the third codon.

Explain how each mutation is likely to affect the primary structure of the polypeptide product, and why the effects of the two mutations differ.

(Note: The genetic code is degenerate and uses triplets of bases to code for amino acids, with some triplets acting as stop codons.)

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6 marks, so the mark scheme is looking for about 6 separate creditworthy points. Making the same point twice in different words earns it once.

Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.

Nucleic acids and protein synthesis: explain question

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