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

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 1.4 DNA, RNA 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. A point mutation changes a single base in a gene.

Model answer (5 marks)

The DNA sequence determines the amino‑acid sequence of a protein. A point mutation changes one base, which can alter the codon read during translation. This codon may now specify a different amino acid, so the polypeptide chain has a wrong residue. The altered amino‑acid sequence changes the way the chain folds, giving a different three‑dimensional shape. Because the shape of the protein is altered, its functional sites (e.g. an enzyme’s active site or a structural motif) no longer fit their normal partners, so the protein becomes non‑functional.

Examiner tips

  • Use the chain of cause and effect: DNA → codon → amino acid → sequence → folding → function.
  • Show the specific link that a wrong amino acid changes the protein’s shape and thus its activity.

Common mistakes

  • Failing to mention that the codon change leads to a different amino acid.
  • Confusing the mutation with a deletion or insertion; only a point mutation is considered.
  • Not linking the altered shape to loss of function.

Mark scheme (5 marks)

  1. The sequence of bases in DNA codes for the sequence of amino acids in a protein
  2. A changed base sequence leads to a different (or wrong) amino acid being incorporated into the protein
  3. This results in a different sequence of amino acids (in the polypeptide chain)
  4. The protein folds into a different three-dimensional shape / the shape of the protein changes
  5. A changed shape means the protein cannot carry out its function (e.g. an enzyme's active site no longer fits its substrate / a structural protein cannot provide support)

Key terms in this question

mutation · protein

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