A student states: 'DNA is found in the nucleus of eukaryotic cells. Before a cell can divide by mitosis, the DNA must be copied so that each daughter cell receives identical genetic information.' Explain why it is important that both daughter cells receive an identical copy of the DNA, and describe the role of complementary base pairing in producing those identical copies.

Eduqas A-Level Biology — 1.5 Nucleic acids and DNA replication · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

DNA is a polymer made of two strands twisted around each other to form a double helix. Each strand is made up of nucleotides, and the bases on one strand are complementary to the bases on the opposite strand.

Model answer (5 marks)

1. Both daughter cells must contain the same genetic information as the parent cell so that they can produce the same proteins and function correctly.
2. The two strands of the double helix separate and unwind.
3. Each original strand acts as a template for a new complementary strand.
4. Complementary base pairing ensures that the correct base is added to each position on the new strand.
5. The result is two identical DNA molecules, each containing one original strand and one new strand.

Examiner tips

  • Use the exact wording from the mark scheme; include all five points. Show the sequence: separation → template → complementary base pairing → identical molecules. Keep sentences short and to the point.

Common mistakes

  • Omitting the need for identical genetic information. Failing to mention that each strand acts as a template. Confusing the role of base pairing with the unwinding step.

Mark scheme (5 marks)

  1. Each daughter cell must contain the same genetic information / genes as the parent cell so that it can produce the same proteins / function correctly
  2. The two strands of the double helix separate / unwind
  3. Each original strand acts as a template for a new complementary strand
  4. Complementary base pairing ensures that the correct base is added to each position on the new strand
  5. The result is two identical DNA molecules, each containing one original strand and one new strand

Key terms in this question

DNA · complementary base pairing · daughter cells · genetic information

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