Explain how the process of transcription produces a molecule of messenger RNA (mRNA) from a gene, and state one way in which mRNA differs in structure from DNA.

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)

1. The DNA double helix unwinds at the gene, separating the two strands.
2. Free RNA nucleotides are attracted to the exposed template strand of DNA.
3. Complementary base‑pairing occurs: A‑U, T‑A, G‑C, C‑G between RNA and the DNA template.
4. RNA nucleotides are joined by phosphodiester bonds to form a single‑stranded mRNA molecule.
5. mRNA differs from DNA in that it is single‑stranded and contains uracil instead of thymine.

Examiner tips

  • Use the sequence of steps in the correct order; each step is worth one mark. Mention the base‑pairing rule (A‑U, T‑A, G‑C, C‑G) to gain the mark for complementary pairing. State a clear structural difference such as uracil vs thymine to secure the final mark.

Common mistakes

  • Writing the steps out of order or omitting the unwinding step. Failing to mention the base‑pairing rule or the presence of uracil. Using plural “mRNAs” or “DNA strands” incorrectly.

Mark scheme (5 marks)

  1. DNA double helix / two strands unzip / separate (at the gene)
  2. Free RNA nucleotides are attracted to / align with the template / complementary strand of DNA
  3. Complementary base pairing occurs (between RNA nucleotides and the DNA template strand)
  4. RNA nucleotides are joined together to form a single-stranded mRNA molecule
  5. mRNA is single-stranded whereas DNA is double-stranded / mRNA contains uracil (instead of thymine) / mRNA is shorter than a DNA molecule / mRNA contains ribose sugar whereas DNA contains deoxyribose

Key terms in this question

transcription · messenger RNA (mRNA)

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