During the cell cycle, DNA replication occurs in the nucleus before a cell can divide by mitosis. Explain how DNA replication produces two identical copies of DNA from one original molecule.

WJEC A-Level Biology (Wales) — 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).

A cell in the root tip of a plant is about to undergo mitosis. Before this can happen, the cell must replicate its DNA during interphase.

Model answer (5 marks)

The double helix unwinds and the two strands separate.
Each original strand acts as a template.
New complementary bases pair with the bases on each template strand.
Two new double‑stranded DNA molecules are formed.
Each new molecule contains one original strand and one newly synthesised strand, so both copies are identical to the original.

Examiner tips

  • Use the word "unwind" or "separate" for the first point. Mention "template strand" and "complementary bases". Show that each new strand is a hybrid of old and new. Keep the answer concise – 5 points, one sentence each.

Common mistakes

  • Confusing the order of steps (e.g. saying synthesis before unwinding). Forgetting to state that each new molecule has one original strand. Using vague terms like "copy" instead of "template" and "complementary bases".

Mark scheme (5 marks)

  1. The two strands of the DNA double helix unwind / separate
  2. Each original strand acts as a template for a new strand
  3. New complementary bases pair up with the bases on each template strand
  4. Two new double-stranded DNA molecules are formed
  5. Each new DNA molecule contains one original (template) strand and one newly synthesised strand, so both copies are identical to the original

Key terms in this question

DNA · DNA replication

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