A molecule of messenger RNA (mRNA) is produced during transcription. Explain how the sequence of bases in a gene determines the primary structure of a polypeptide, referring to the roles of mRNA and transfer RNA (tRNA) in this process.

OCR A-Level Biology A (H420) — 2.3 Nucleotides and nucleic acids · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The base sequence of the gene (template strand) is transcribed to produce a complementary mRNA sequence, so the sequence of codons on mRNA reflects the original DNA base sequence.
Each codon (triplet of bases) on the mRNA codes for a specific amino acid.
tRNA molecules carry specific amino acids and have a complementary anticodon that base‑pairs with the mRNA codon at the ribosome.
Amino acids are joined by peptide bonds in a sequence determined by the mRNA, forming the primary structure of the polypeptide.

Examiner tips

  • Use the word ‘transcribed’ and ‘complementary’ to link DNA to mRNA. Mention codons and anticodons explicitly. Show the flow: DNA → mRNA → tRNA → polypeptide. Include the role of peptide bonds in linking amino acids.

Common mistakes

  • Confusing the coding strand with the template strand. Forgetting to mention that tRNA carries the amino acid. Not specifying that peptide bonds link the amino acids in the order given by the mRNA.

Mark scheme (4 marks)

  1. The base sequence of the gene (template strand) is transcribed to produce a complementary mRNA sequence, so the sequence of codons on mRNA reflects the original DNA base sequence
  2. Each codon (triplet of bases) on the mRNA codes for a specific amino acid
  3. tRNA molecules carry specific amino acids and have a complementary anticodon that base-pairs with the mRNA codon at the ribosome
  4. Amino acids are joined by peptide bonds in a sequence determined by the mRNA, forming the primary structure of the polypeptide

Key terms in this question

messenger RNA (mRNA) · transcription · transfer RNA (tRNA) · primary structure

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