Explain why DNA replication is described as semi-conservative.

IB DP Biology Standard 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).

During DNA replication, each strand of the double helix serves as a template for the synthesis of a new complementary strand. The term 'semi-conservative' was confirmed experimentally by Meselson and Stahl in 1958 using nitrogen isotope labelling.

Model answer (4 marks)

1. The parental double helix is unwound, separating the two strands.
2. Each original strand serves as a template for synthesising a new complementary strand.
3. Each daughter DNA molecule therefore contains one parental strand and one newly‑made strand.
4. Because half of each new molecule is conserved from the original, the replication is called semi‑conservative.

Examiner tips

Use the exact phrase ‘semi‑conservative’ and explain that half of each new molecule is conserved.
Show the stepwise logic: unwinding → template → new strand → half‑conserved.

Common mistakes

Confusing ‘conservative’ with ‘semi‑conservative’ or omitting the ‘half’ part.
Failing to mention that each daughter molecule contains one parental strand.

Mark scheme (4 marks)

The two strands of the parental double helix are separated / unwound prior to replication.
Each original (parental) strand acts as a template for the synthesis of a new complementary strand.
Each resulting daughter DNA molecule contains one original (parental) strand and one newly synthesised strand.
The process is 'semi-conservative' because half of each new DNA molecule is conserved from the original / parental molecule.

Key terms in this question

semi-conservative · DNA replication