Students should be able to relate the nature of a gene mutation to its…

The idea

This is a reasoning skill: given a mutation, work out what happens to the protein by asking three questions in order. First, does the reading frame survive? A substitution, or an addition or deletion of a multiple of three bases (for example three or six), preserves the frame; an addition or deletion of any number of bases that is not a multiple of three shifts it, so every downstream triplet changes and the polypeptide is almost certainly non-functional. Second, if the frame survives, does the amino acid actually change? Because the code is degenerate, the new triplet may specify the same amino acid, in which case nothing changes at all. Third, if one amino acid has changed, where in the molecule is it? A change in the active site matters more than one on the surface.

What earns the marks

• Identifying the type of mutation from a given sequence or description, for example substitution, addition or deletion.
• Using the degeneracy of the code where the amino acid is unchanged, for example a substitution producing a triplet that still codes for the same amino acid.
• Linking a changed amino acid to altered R group interactions and a changed tertiary structure.
• A correct statement about function, such as no enzyme-substrate complexes forming, or a truncated polypeptide from a premature stop codon.

Common mistakes and how to put them right

• Jumping straight to the protein without first identifying the type of mutation.

  Put it right: Name the mutation type first, then work out its effect.

• Assuming every mutation is harmful, when silent and neutral mutations are common.

  Put it right: Check whether the amino acid sequence actually changes before claiming an effect.

• Comparing two DNA sequences base by base and missing that an insertion has shifted everything along.

  Put it right: Align the sequences in triplets from the start of the gene.

• Forgetting to check whether a new stop triplet has been created.

  Put it right: Scan the new sequence for stop triplets, as these truncate the polypeptide.

• Writing about the effect on the gene when the question asked about the polypeptide.

  Put it right: Answer in terms of amino acid sequence, tertiary structure and protein function.

Exam tips

• Work the sequence through in triplets on the paper; examiners credit correct working even when the final statement is incomplete.
• Quantify the change: say one amino acid changed, or all amino acids after position 12 changed.
• Name the mutation type first, then the effect; the two are usually separate marks.