During the cell cycle, DNA replication occurs in the nucleus before a cell can divide. Explain how DNA replication produces two identical copies of DNA from one original molecule.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. The double helix unwinds and the two strands separate.
2. Each original strand acts as a template for a new complementary strand.
3. Free nucleotides align with complementary bases on the template strand and join to form the new strand.
4. Two new double‑stranded DNA molecules are formed, each containing one original strand and one newly synthesised strand.
5. The two new DNA molecules are identical to the original, so each daughter cell receives identical genetic information.
2. Each original strand acts as a template for a new complementary strand.
3. Free nucleotides align with complementary bases on the template strand and join to form the new strand.
4. Two new double‑stranded DNA molecules are formed, each containing one original strand and one newly synthesised strand.
5. The two new DNA molecules are identical to the original, so each daughter cell receives identical genetic information.
Examiner tips
- Use the word ‘unwind’ and ‘separate’ for step 1. Mention ‘template strand’ and ‘complementary bases’ for step 2. Show that new strands are formed by base‑pairing. State that each new molecule has one old and one new strand. Finish by noting identical genetic information for daughter cells.
Common mistakes
- Forgetting to mention the unwinding of the helix. Saying that both strands are new rather than one old and one new. Using vague terms like ‘copy’ instead of ‘template strand’ and ‘complementary bases’.
Mark scheme (5 marks)
- The two strands of the double helix unwind / separate
- Each original strand acts as a template for a new strand
- Complementary bases pair up / free nucleotides align with complementary bases on the template strand
- Two new double-stranded DNA molecules are formed, each containing one original strand and one new strand
- The two new DNA molecules are identical to the original / each daughter cell receives identical genetic information
Key terms in this question
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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