Describe the structure of a DNA molecule and explain how this structure allows DNA to be copied accurately during cell division.

AQA A-Level Biology (7402) — 3.1.5 Nucleic Acids · Describe and Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

DNA consists of two antiparallel strands of nucleotides twisted into a right‑handed double helix. Each strand is a sugar‑phosphate backbone with attached bases: adenine, thymine, cytosine and guanine. The strands are held together by complementary base‑pairing – A pairs with T via two hydrogen bonds and C pairs with G via three. During replication the strands separate (unzipping) and each becomes a template. Free nucleotides in the cell then pair with the exposed bases (A with T, C with G) to form two new complementary strands, producing two identical DNA molecules.

Examiner tips

  • Use the phrase "antiparallel strands" and "right‑handed double helix" to show structure. Mention base‑pairing with hydrogen bonds. Explain unzipping and template role for accuracy. Keep answer concise and use correct terminology.

Common mistakes

  • Confusing A with G or T with C; not specifying antiparallel orientation; omitting the role of complementary base‑pairing in accurate copying.

Mark scheme (5 marks)

  1. DNA is made up of two strands
  2. The strands are twisted into a double helix
  3. The strands are held together by bases / base pairs (adenine pairs with thymine; cytosine pairs with guanine)
  4. During copying, the two strands separate / unzip
  5. Each strand acts as a template so that complementary/free bases pair up to produce two identical copies

Key terms in this question

DNA

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