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. The bases pair by complementary hydrogen bonds: adenine with thymine, cytosine with guanine.
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. The bases pair by complementary hydrogen bonds: adenine with thymine, cytosine with guanine.
Examiner tips
- Use the exact terms: double‑stranded, double helix, nucleotides, complementary base pairing.
- Show the base pairs A‑T and C‑G to earn the full 4 marks.
Common mistakes
- Writing single‑stranded DNA or a left‑handed helix.
- Omitting the phosphate‑sugar backbone or the base‑pairing detail.
Mark scheme (4 marks)
- DNA is made up of two strands / is double-stranded
- The two strands are twisted / coiled to form a double helix
- Each strand is made up of nucleotides / the molecule is made of nucleotides
- Each nucleotide contains a base, and the bases pair up / complementary base pairing holds the two strands together (A with T; C with G)
Key terms in this question
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →