Explain why a strand of mRNA produced during transcription is complementary to, but not identical to, the DNA template strand from which it was made, yet has the same base sequence as the non-template (coding) strand of DNA, with one exception. Identify that exception and explain its molecular basis.

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, the enzyme RNA polymerase moves along one strand of DNA, the template strand, synthesising a single-stranded mRNA molecule. The mRNA then moves to ribosomes where it is translated into a polypeptide.

Model answer (4 marks)

mRNA is complementary to the template strand because RNA polymerase adds ribonucleotides that base‑pair with the template DNA.

Since the two DNA strands are complementary, the mRNA sequence therefore matches the non‑template (coding) strand.

The exception is that the mRNA contains uracil (U) where the coding strand contains thymine (T).

This occurs because RNA uses uracil instead of thymine; uracil lacks the methyl group that thymine has, but still pairs with adenine.

Examiner tips

  • Show the complementary base‑pairing logic first, then explain the U/T switch; use the words ‘complementary’, ‘coding strand’, ‘uracil’, ‘thymine’.
  • Keep the answer to four clear points, matching the mark scheme structure.
  • Mention the ribose vs deoxyribose difference only if time allows – it is not required for full marks.

Common mistakes

  • Confusing the template strand with the coding strand; students write that mRNA is identical to the template.
  • Forgetting to state that uracil replaces thymine, or misidentifying the exception as a base‑pairing error.

Mark scheme (4 marks)

  1. mRNA is complementary to the template strand because RNA polymerase adds free ribonucleotides by complementary base pairing with the template strand
  2. Because the mRNA sequence is complementary to the template strand, it therefore has the same sequence as the non-template (coding) strand, since the two DNA strands are themselves complementary to each other
  3. The exception is that mRNA contains uracil (U) where the coding strand of DNA contains thymine (T)
  4. The molecular basis is that RNA nucleotides contain ribose sugar rather than deoxyribose, and uracil (not thymine) is the pyrimidine incorporated — uracil lacks the methyl group present on thymine but still base-pairs with adenine

Key terms in this question

mRNA · template strand · transcription · complementary · non-template (coding) strand

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