A gene is made up of a sequence of DNA bases. Explain how the sequence of bases in a gene determines the structure of a specific protein.

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).

Model answer (5 marks)

The DNA sequence of a gene is read in groups of three bases, called codons, each of which specifies a particular amino acid. During transcription the DNA strand is copied into mRNA, with U replacing T. The mRNA then travels to a ribosome where translation occurs: transfer RNA brings the corresponding amino acids to the ribosome in the order dictated by the codons. The amino acids are linked together by peptide bonds, forming a polypeptide chain whose sequence determines its three‑dimensional shape and therefore its function.

Examiner tips

  • Use the phrase ‘codon specifies an amino acid’ to show understanding of the triplet rule.
  • Mention transcription (DNA→mRNA) and translation (mRNA→protein) to cover all steps.
  • Explain that the amino‑acid sequence determines the protein’s structure and function.

Mark scheme (5 marks)

  1. The sequence of bases in DNA codes for the sequence of amino acids (in a protein)
  2. Every three bases (a triplet / codon) codes for one amino acid
  3. A copy of the gene is made as mRNA (transcription) / the DNA code is copied into mRNA
  4. mRNA moves to a ribosome where the protein is assembled (translation)
  5. Amino acids are joined together in a specific sequence to form a protein / the order of amino acids determines the shape and function of the protein

Key terms in this question

gene · DNA bases · protein

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