Chromosomes are long, coiled molecules of DNA. DNA is a polymer made of two strands twisted around each other to form a double helix. Explain how the structure of DNA allows it to be replicated accurately during the cell cycle.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Before a eukaryotic cell can divide by mitosis, DNA replication must occur during interphase so that each daughter cell receives an identical copy of the genetic material.
Model answer (5 marks)
DNA is a double helix made of two complementary strands.
1. The strands unwind and separate during replication.
2. Each single strand then acts as a template.
3. DNA polymerase adds nucleotides that pair with the template via complementary base‑pairing (A‑T, G‑C).
4. This produces a new strand that is identical to the template.
5. The result is two DNA molecules, each containing one original and one newly synthesised strand, so the genetic information is copied accurately.
1. The strands unwind and separate during replication.
2. Each single strand then acts as a template.
3. DNA polymerase adds nucleotides that pair with the template via complementary base‑pairing (A‑T, G‑C).
4. This produces a new strand that is identical to the template.
5. The result is two DNA molecules, each containing one original and one newly synthesised strand, so the genetic information is copied accurately.
Examiner tips
- Use the term ‘double helix’ and ‘two strands’ early. Show the sequence: unwind → template → complementary base‑pairing → identical strands. Mention DNA polymerase only if time allows, but it is not required for full marks.
Common mistakes
- Forgetting to state that the strands are complementary. Confusing ‘unzip’ with ‘duplicate’ – the strands separate before new strands are made. Leaving out the fact that each new molecule contains one original strand.
Mark scheme (5 marks)
- DNA is made of two strands / is a double helix
- The two strands of DNA unwind / separate / unzip
- Each separated strand acts as a template for a new strand
- Complementary base pairing ensures the new strand is identical to the original / bases pair in a specific way
- Two identical DNA molecules are produced, each containing one original and one new strand
Key terms in this question
DNA · double helix · polymer · chromosome · cell cycle
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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