Explain why a molecule of messenger RNA (mRNA) produced during transcription is described as complementary to the template strand of DNA, and explain why the base sequence of mRNA is identical to the coding strand of DNA rather than complementary to it.

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).

During gene expression, a gene is first transcribed into mRNA before translation occurs. The DNA double helix has two strands: the template strand, which is read by RNA polymerase, and the coding strand (also called the sense strand), which has the same sequence as the mRNA produced.

Model answer (4 marks)

RNA polymerase reads the template strand and adds nucleotides by complementary base pairing.
Adenine pairs with uracil (and cytosine with guanine), so each base in mRNA is the complement of the corresponding base on the template strand.
The coding strand has the same base sequence as the template strand’s complement, so it shares the same sequence as the mRNA (with T replaced by U).
Therefore the mRNA carries the same genetic information as the coding strand, allowing the correct amino acid sequence to be specified during translation.

Examiner tips

  • Use the phrase ‘complementary base pairing’ to show understanding of the mechanism.
  • Show the A↔U, C↔G rule explicitly.
  • Explain that the coding strand is the reverse complement of the template strand.
  • Link the identical sequence to translation accuracy.

Common mistakes

  • Confusing the coding strand with the template strand in the explanation.
  • Forgetting to mention that uracil replaces thymine in mRNA.
  • Using vague terms like ‘same’ without specifying base‑pairing rules.

Mark scheme (4 marks)

  1. RNA polymerase reads the template strand and adds nucleotides by complementary base pairing
  2. Adenine pairs with uracil (and cytosine with guanine), so each base in mRNA is the complement of the corresponding base on the template strand
  3. The coding strand has the same base sequence as the template strand's complement, so it shares the same sequence as the mRNA (with T replaced by U)
  4. Therefore the mRNA carries the same genetic information as the coding strand, allowing the correct amino acid sequence to be specified during translation

Key terms in this question

messenger RNA (mRNA) · template strand · transcription · coding strand

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