A mutation occurs in a gene, changing one DNA base. Explain how this mutation could result in a non-functional protein being produced.

AQA A-Level Biology (7402) — 3.4.2 DNA and Protein Synthesis · Explain · 5 marks · View as Markdown

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

Mutations are changes to the sequence of bases in DNA. Some mutations alter the protein that is produced from that gene.

Model answer (5 marks)

A mutation changes the sequence of bases in the DNA of the gene.
The altered DNA is transcribed to produce a different mRNA sequence.
The changed mRNA codon now codes for a different amino acid.
The sequence of amino acids in the protein is altered.
The altered amino‑acid sequence changes the protein’s folding, so the active site or overall shape is altered and the protein becomes non‑functional.

Examiner tips

Use the word ‘mutation’ first, then explain the chain of events: DNA → mRNA → amino acid → protein structure. Show the link between a codon change and a different amino acid. Mention that the altered sequence can disturb the protein’s 3‑D shape or active site.
Use the exact terminology: ‘transcribed’, ‘codon’, ‘amino acid’, ‘folds’, ‘active site’.
Keep the answer concise – 5 points, one sentence per point.

Common mistakes

Confusing the mutation with a deletion or insertion; only a single base change is required. Failing to mention that the altered codon changes the amino acid. Not linking the altered amino‑acid sequence to a change in protein folding or function.

Mark scheme (5 marks)

A mutation changes the sequence of bases in the DNA / gene
The altered DNA is transcribed to produce a different / altered mRNA sequence
The changed mRNA codon codes for a different amino acid
The sequence of amino acids in the protein is changed / different
The protein folds into a different shape so it cannot function / active site shape is changed so it cannot work

Key terms in this question

mutation · protein