ACT SSC Biology — Unit 2
DNA Replication — Flashcards & Quiz
ACT SSC Biology Year 12 Unit 2 covers DNA replication as the semi-conservative process that copies the genome before cell division. You need to name the enzymes, explain leading vs lagging strand synthesis, and describe how proofreading maintains fidelity. The Meselson-Stahl experiment provides the historical evidence.
Key Points
- DNA replication is semi-conservative: each new double helix contains one parental strand and one newly synthesised strand.
- Helicase unwinds the double helix; single-strand binding proteins prevent re-annealing.
- DNA polymerase synthesises new DNA 5'→3', reading the template 3'→5'.
- Leading strand: continuous synthesis toward the replication fork.
- Lagging strand: discontinuous synthesis in short Okazaki fragments, joined by DNA ligase.
- Proofreading: DNA polymerase has 3'→5' exonuclease activity that corrects incorporation errors.
Common Mistakes to Avoid
- Forgetting that DNA polymerase only works 5'→3' — this is why the lagging strand uses Okazaki fragments.
- Confusing helicase (unwinds) with DNA polymerase (copies).
- Describing replication as "conservative" or "dispersive" instead of semi-conservative.
- Omitting DNA ligase from the enzyme list — it's required to join Okazaki fragments.
- Drawing replication as unidirectional — it proceeds bidirectionally from origins.
Exam Strategy
BSSS Unit 2 DNA replication questions ask you to describe the process with enzyme specificity. Structure: (1) origin of replication, (2) helicase unwinds, (3) DNA polymerase synthesises both strands, (4) ligase joins Okazaki fragments, (5) proofreading catches errors. Label a replication fork diagram clearly.
Sample Flashcards
Q1: Outline the process of DNA replication.
Helicase unwinds and separates strands. DNA polymerase adds complementary nucleotides to each template strand (5' to 3'). Leading strand is continuous; lagging strand is made in Okazaki fragments joined by ligase.
Sample Quiz Questions
Q1: DNA polymerase synthesises new DNA strands in the 3' to 5' direction.
Answer: FALSE
DNA polymerase adds nucleotides in the 5' to 3' direction.
Revision Tip
Enzyme-function pairs are high-yield flashcard content — drill helicase, DNA polymerase, ligase, and their roles until recall is automatic.
Last updated: March 2026 · 1 flashcards · 1 quiz questions