CAIE A Level Biology 9700 · Nucleic acids and protein synthesis

Nucleic acids and protein synthesis: explain question

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A short segment of a bacterial gene that codes for a short polypeptide is shown below. The two strands of the DNA double helix are labelled P and Q, with their polarities indicated.

Strand P (5' → 3'): ATG GGC CTA TCA GAT ACT Strand Q (3' → 5'): TAC CCG GAT AGT CTA TGA

The start codon for the polypeptide is ATG on the coding strand.

(a) Identify which strand (P or Q) is the template strand. Explain your reasoning. (b) A deletion mutation removes the second base (the middle base) of the second codon on the coding strand. Explain how this mutation would affect the amino acid sequence of the polypeptide produced from this gene.

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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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