Explain why DNA replication is described as semi-conservative and outline the significance of this mechanism for inheritance.

IB DP Biology Higher Level (2023 syllabus) — D1.1 DNA replication · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Each new double helix contains one original (parental) strand and one newly synthesised strand
The parental strand acts as a template, with complementary base pairing directing the sequence of the new strand
The base sequence of the parental DNA is faithfully reproduced in both daughter molecules, preserving genetic information
Semi‑conservative replication ensures that heritable information is transmitted unchanged across cell generations, underpinning genetic continuity

Examiner tips

  • Use the phrase "semi‑conservative" early to show understanding
  • Explain the template role and base‑pairing clearly
  • Mention faithful reproduction of sequence
  • Link to inheritance and genetic continuity

Common mistakes

  • Confusing conservative or dispersive replication with semi‑conservative
  • Failing to state that each daughter helix has one parental strand
  • Omitting the significance for inheritance

Mark scheme (4 marks)

  1. Each new double helix contains one original (parental) strand and one newly synthesised strand
  2. The parental strand acts as a template, with complementary base pairing directing the sequence of the new strand
  3. The base sequence of the parental DNA is faithfully reproduced in both daughter molecules, preserving genetic information
  4. Semi-conservative replication ensures that heritable information is transmitted unchanged across cell generations / underpins genetic continuity

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