A scientist studies DNA replication in human liver cells. Explain how a single molecule of DNA produces two identical copies of itself during DNA replication.

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).

Human liver cells must replicate their DNA during the cell cycle before cell division can occur. DNA replication takes place in the nucleus and ensures that each new daughter cell receives a full set of genetic information.

Model answer (5 marks)

DNA double‑helix unwinds and the two strands separate.
Each original strand serves as a template.
New nucleotides are added by complementary base‑pairing to the exposed bases on the template strand.
Two new double‑stranded DNA molecules are formed.
Each new molecule contains one original strand and one newly synthesised strand (semi‑conservative replication).

Examiner tips

Use the exact phrase ‘semi‑conservative replication’ to secure the final mark.
Show the stepwise process: unwind → template → base‑pairing → new strands.
Keep the answer concise, 5 short points, no extra wording.

Common mistakes

Writing ‘conservative’ instead of ‘semi‑conservative’. Failing to mention that each new molecule contains one old strand. Using vague terms like ‘copy’ instead of ‘template’ and ‘base‑pairing’.

Mark scheme (5 marks)

DNA double helix unwinds / two strands separate
Each original strand acts as a template for a new strand
Complementary base pairing occurs — new bases pair with exposed bases on the template strand
Two new identical double-stranded DNA molecules are formed
Each new DNA molecule contains one original strand and one new strand (semi-conservative replication)

Key terms in this question

DNA