Explain how the base pairing rule ensures that the two strands of a DNA molecule are complementary to each other.

WJEC GCSE Biology (Wales) — 2.3.1 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)

Each base on one strand pairs with a specific base on the other strand.
Adenine pairs with thymine (A–T).
Cytosine pairs with guanine (C–G).
Because the base sequence on one strand determines the base sequence on the other strand, the strands are complementary.

Examiner tips

  • Use the exact base‑pairing terms (A–T, C–G).
  • Show the logical sequence: specific pairing → fixed sequence → complementarity.
  • Keep sentences short and to the point.

Mark scheme (4 marks)

  1. Each base on one strand pairs with a specific base on the other strand
  2. Adenine pairs with thymine (A–T)
  3. Cytosine pairs with guanine (C–G)
  4. Because the base sequence on one strand determines/fixes the base sequence on the other strand, the strands are complementary

Key terms in this question

base pairing rule · complementary

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