QCE Biology — Unit 4
DNA Replication — Flashcards & Quiz
DNA replication is the semi-conservative copying of the genome and a core QCE Biology Unit 4 concept. You need to name the key enzymes, explain why leading and lagging strand synthesis differ, and describe how proofreading maintains fidelity. The Meselson-Stahl experiment provides the classic evidence that replication is semi-conservative, and is a favourite exam context.
Key Points
- DNA replication is semi-conservative: each new molecule contains one parental strand and one newly synthesised strand.
- Helicase unwinds the double helix; single-strand binding proteins keep strands apart.
- DNA polymerase synthesises new DNA 5' to 3', reading the template 3' to 5'.
- Leading strand: continuous synthesis toward the fork. Lagging strand: discontinuous Okazaki fragments joined later by ligase.
- Proofreading: DNA polymerase checks each new base and removes incorrect ones; mismatch repair catches remaining errors.
- Meselson-Stahl confirmed semi-conservative replication using ¹⁵N/¹⁴N density gradients.
Common Mistakes to Avoid
- Forgetting that DNA polymerase only works 5'→3' — this is why the lagging strand needs Okazaki fragments.
- Confusing helicase with DNA polymerase — helicase unwinds, polymerase copies.
- Describing replication as "conservative" or "dispersive" — Meselson-Stahl showed it is semi-conservative.
- Omitting DNA ligase — it joins Okazaki fragments on the lagging strand.
- Drawing replication as unidirectional — it proceeds bidirectionally from origins.
Exam Strategy
QCAA Unit 4 DNA replication questions ask you to describe the process and name the enzymes. Structure: origin of replication → helicase unwinds → primase lays primers → DNA polymerase synthesises leading strand continuously and lagging strand in Okazaki fragments → ligase joins the fragments → proofreading catches errors. Label a diagram of the replication fork clearly with enzymes and strand directions.
Sample Flashcards
Q1: Outline the process of semi-conservative DNA replication.
Helicase unwinds the double helix and breaks hydrogen bonds between base pairs, forming a replication fork. DNA polymerase III adds complementary nucleotides to each template strand in the 5' to 3' direction. The leading strand is synthesised continuously; the lagging strand is synthesised in Okazaki fragments joined by DNA ligase. Each new molecule contains one original and one new strand (semi-conservative).
Sample Quiz Questions
Q1: DNA replication is described as semi-conservative because each new DNA molecule contains one original strand and one new strand.
Answer: TRUE
Semi-conservative means each daughter molecule retains (conserves) one of the original parent strands and pairs it with a newly synthesised complementary strand.
Q2: DNA polymerase can add nucleotides in both the 5' to 3' and 3' to 5' directions.
Answer: FALSE
DNA polymerase can ONLY add nucleotides in the 5' to 3' direction. This is why the lagging strand must be synthesised in short Okazaki fragments.
Revision Tip
Enzyme-function pairs are classic Revizi flashcard content — drill helicase, primase, DNA polymerase, ligase and their roles until you can recite them in under 30 seconds.
Last updated: March 2026 · 1 flashcards · 2 quiz questions