Describe the structure of a DNA molecule.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
DNA is a double‑stranded molecule.
1. The two strands are twisted into a right‑handed double helix.
2. Each strand is a chain of nucleotides.
3. A nucleotide consists of a phosphate, a deoxyribose sugar and a nitrogenous base.
4. Bases on opposite strands are held together by hydrogen bonds: A pairs with T and C pairs with G.
1. The two strands are twisted into a right‑handed double helix.
2. Each strand is a chain of nucleotides.
3. A nucleotide consists of a phosphate, a deoxyribose sugar and a nitrogenous base.
4. Bases on opposite strands are held together by hydrogen bonds: A pairs with T and C pairs with G.
Examiner tips
- Use the exact terms: double‑stranded, double helix, nucleotides, base pairing (A–T, C–G).
- Show the four points in a logical order – structure, strands, nucleotides, base pairing.
- Keep the answer concise; each point counts as one mark.
Common mistakes
- Confusing DNA with RNA (e.g., using ribose or uracil).
- Omitting the double‑helix twist or the base‑pairing detail.
Mark scheme (4 marks)
- DNA is made up of two strands / double-stranded
- The two strands are twisted into a double helix
- Each strand is made up of nucleotides / the monomers/units are nucleotides
- Each nucleotide contains a base, and the bases on opposite strands are paired / held together by base pairing (A–T and C–G)
Key terms in this question
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →