Explain how a molecule of DNA in a cell nucleus controls the sequence of amino acids in a protein that is assembled in the cytoplasm.

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)

DNA in the nucleus contains a specific sequence of bases that encodes a protein. The DNA sequence is transcribed into a complementary mRNA strand. The mRNA exits the nucleus and binds to a ribosome in the cytoplasm. The ribosome reads the mRNA three bases at a time, each set of three bases (a codon) specifying one amino acid. Transfer RNA (tRNA) molecules, each carrying a particular amino acid, match their anticodon to the mRNA codon and deliver the amino acid to the ribosome, so the base sequence determines the amino‑acid sequence of the protein.

Examiner tips

  • Use the word ‘transcribed’ for DNA to mRNA, ‘translated’ for mRNA to protein.
  • Show the flow: DNA → mRNA → ribosome → tRNA → amino acids.
  • Mention codons and the role of tRNA anticodons.
  • Keep the answer concise and use correct terminology.

Common mistakes

  • Confusing transcription with translation or vice versa.
  • Omitting the role of tRNA or the codon‑anticodon matching.
  • Using vague terms like ‘copy’ instead of ‘transcribe’ or ‘assemble’ instead of ‘translate’.

Mark scheme (5 marks)

  1. DNA / gene contains a sequence of bases (that codes for a protein)
  2. The base sequence is transcribed / copied into mRNA
  3. mRNA moves out of the nucleus / to the ribosome (in the cytoplasm)
  4. Each three bases (codon) on the mRNA codes for one specific amino acid
  5. tRNA carries / brings the correct amino acid to the ribosome, so the sequence of bases determines the sequence of amino acids in the protein

Key terms in this question

DNA · amino acid

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