Explain how the sequence of bases in a gene determines the sequence of amino acids in a protein.

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).

Protein synthesis occurs in two main stages: transcription in the nucleus and translation at the ribosome. The order of amino acids in a protein determines its shape and function.

Model answer (5 marks)

DNA contains a sequence of bases that codes for a protein.
During transcription the DNA sequence is used as a template to synthesise mRNA.
The mRNA carries the genetic code out of the nucleus to the ribosome.
At the ribosome each set of three bases (a codon) on the mRNA specifies one amino acid.
Amino acids are joined together in the order dictated by the mRNA sequence to form the protein.

Examiner tips

  • Use the exact terminology: DNA, gene, transcription, mRNA, ribosome, codon, translation.
  • Show the step‑by‑step flow: DNA → mRNA → ribosome → amino acid chain.
  • Keep the answer concise – each point should be a short sentence.

Mark scheme (5 marks)

  1. DNA / gene contains a sequence of bases (that codes for a protein)
  2. The sequence of bases in DNA is used as a template to make mRNA during transcription
  3. mRNA carries the code out of the nucleus to the ribosome
  4. At the ribosome, every three bases (a codon / triplet) on the mRNA codes for one amino acid
  5. Amino acids are joined together in the order determined by the mRNA sequence to form a protein (translation)

Key terms in this question

amino acid

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