A student says: 'DNA is just a long chain of nucleotides joined together.' Explain why this description is incomplete.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
DNA is not a single chain but two strands that form a double helix.
The strands are twisted around each other.
They are joined by complementary base pairs.
A pairs with T and C pairs with G.
The strands are twisted around each other.
They are joined by complementary base pairs.
A pairs with T and C pairs with G.
Examiner tips
- Use the exact wording from the mark scheme (e.g., "two strands", "double helix", "complementary base pairs", "A–T, C–G").
- Show the logical sequence: number of strands → shape → bonding → base‑pair rules.
- Keep each point brief but complete – one sentence per point is enough for 4 marks.
Common mistakes
- Saying DNA is a single chain or ignoring the double‑helix structure.
- Omitting the complementary base‑pairing or the specific A‑T, C‑G rules.
- Using vague terms like "paired” without specifying the bases involved.
Mark scheme (4 marks)
- DNA consists of TWO strands / chains (not one)
- The two strands are twisted into a double helix
- The strands are held together / joined by complementary base pairs
- Specific base pairing: A pairs with T and C pairs with G
Key terms in this question
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →