Explain how a DNA molecule is able to copy itself accurately.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The two strands of the DNA molecule separate, creating a replication fork. Each original strand then acts as a template for a new complementary strand. Free DNA bases in the cell pool pair with the complementary bases on each template strand (A with T, C with G). As a result, two identical DNA molecules are produced, each consisting of one original strand and one newly synthesised strand.
Examiner tips
- Use the phrase "replication fork" to show understanding of strand separation.
- Mention complementary base pairing explicitly (A‑T, C‑G).
- Show that each new molecule contains one old and one new strand to demonstrate semi‑conservative replication.
- Keep the answer concise – 4 marks allow a short paragraph, not a long explanation.
Common mistakes
- Failing to mention that the strands separate before synthesis.
- Omitting the complementary base‑pairing rule.
- Claiming that both strands are new, rather than one old and one new.
Mark scheme (4 marks)
- The two strands of the DNA molecule separate / unzip
- Each original strand acts as a template for a new strand
- Free DNA bases pair up with complementary bases on each template strand
- Two identical DNA molecules are produced / each new molecule contains one original strand and one new strand
Key terms in this question
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →