Explain why a change in the base sequence of a gene may not always result in a change in the amino acid sequence of the protein produced.

AQA A-Level Biology (7402) — 3.1.5 Nucleic Acids · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

A change in the base sequence of a gene may not always change the amino acid sequence because:
1. The genetic code is read in triplets (codons).
2. There are 64 possible codons but only 20 amino acids, so the code is degenerate.
3. A mutation can alter a codon to another that still encodes the same amino acid.
4. Thus the amino acid sequence, and consequently the protein structure and function, remain unchanged.

Examiner tips

Use the word ‘degenerate’ or ‘redundant’ to show understanding of the code.
Show the codon‑triplet concept and give an example of a synonymous mutation.
Explain that unchanged amino acid sequence means unchanged protein function.

Common mistakes

Confusing the number of codons with the number of amino acids.
Failing to mention that codons are read in groups of three.
Assuming every mutation changes the protein, ignoring synonymous mutations.

Mark scheme (5 marks)

The base sequence of a gene codes for the amino acid sequence of a protein
Bases are read / coded in groups of three (triplets / codons)
There are more possible triplet combinations than there are amino acids, so the code is degenerate / redundant
If the change in base sequence produces a triplet that still codes for the same amino acid, the amino acid sequence is unchanged
Therefore the protein produced has the same amino acid sequence / structure / function

Key terms in this question

base sequence · gene · amino acid sequence