Describe the structure of a DNA molecule.

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

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

Model answer (4 marks)

DNA is a double‑stranded polymer.
The two strands are wound to form a right‑handed double helix.
Each strand is a chain of nucleotides – a sugar, a phosphate group and a base (A, T, C or G).
Complementary base pairing (A with T, C with G) via hydrogen bonds holds the strands together.

Examiner tips

Use the exact terms: double‑stranded, double helix, nucleotides, complementary base pairing, hydrogen bonds.
Show the base pairs (A‑T, C‑G) to demonstrate understanding of pairing.
Keep the answer concise – 4 points, no extra detail.

Common mistakes

Confusing single‑stranded DNA or not mentioning the double helix.
Omitting the sugar‑phosphate backbone or the four bases.
Using incorrect base pairs (e.g. A‑C or T‑G).

Mark scheme (4 marks)

DNA is a double-stranded polymer / made of two strands
The two strands are wound / twisted to form a double helix
DNA is made of monomers called nucleotides, each consisting of a sugar, a phosphate group and one of four bases (A, T, C or G)
Complementary base pairing holds the two strands together, with A pairing with T and C pairing with G, via hydrogen bonds

Key terms in this question

DNA