Explain how complementary base pairing contributes to the stability of a DNA molecule and to the accurate copying of genetic information.

Edexcel GCSE Biology (1BI0) — 3.3 DNA structure · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Complementary base pairing means that adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G) by forming weak hydrogen bonds.
The hydrogen bonds between these complementary bases hold the two strands of the double helix together, giving the DNA molecule structural stability.
During replication the double helix unwinds and the two strands separate, each strand acting as a template.
Because a base can only pair with its complementary partner, the sequence of bases is copied accurately, producing two identical DNA molecules.

Examiner tips

  • Use the exact base‑pairing pairs (A‑T, C‑G) and mention hydrogen bonds. Explain both stability and accurate copying. Keep answer concise and structured in clear points.

Common mistakes

  • Mixing up A‑C or G‑T pairs. Forgetting to mention hydrogen bonds. Failing to link base‑pairing to both stability and replication accuracy.

Mark scheme (4 marks)

  1. Complementary base pairing describes how A pairs with T and C pairs with G (weak hydrogen bonds form between the complementary base pairs)
  2. The hydrogen bonds between complementary base pairs hold the two strands of the double helix together, giving the molecule stability
  3. When DNA is replicated, the two strands separate and each strand acts as a template
  4. Because each base can only pair with its complementary base, the sequence of bases is copied accurately, producing two identical DNA molecules

Key terms in this question

complementary base pairing · DNA

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