HSC Biology — Module 5
DNA Replication — Flashcards & Quiz
DNA replication is the semi-conservative process that copies the genome before cell division, and it is a high-frequency exam topic in HSC Biology Module 5. You need to name the key enzymes — helicase, DNA polymerase, ligase — and explain the difference between leading and lagging strand synthesis. Examiners reward enzyme specificity and the ability to link replication accuracy to the prevention of mutation. The Meselson-Stahl experiment is the classic evidence question: be ready to interpret nitrogen-isotope banding patterns.
Key Points
- DNA replication is semi-conservative: each new double helix contains one parent strand and one newly synthesised strand (Meselson-Stahl, 1958).
- Key enzymes: helicase unwinds the double helix, DNA polymerase adds complementary nucleotides 5′→3′, ligase joins Okazaki fragments.
- Leading strand is synthesised continuously toward the replication fork; lagging strand is built in short Okazaki fragments away from the fork.
- Proofreading: DNA polymerase has 3′→5′ exonuclease activity that corrects ~99% of incorporation errors; mismatch repair catches the rest.
- Replication occurs in S phase of interphase, before mitosis, so each chromosome can separate into identical sister chromatids.
- Common HSC trap: confusing the "template" strand (read 3′→5′) with the "coding" strand (sequence matches mRNA except T→U).
Common Mistakes to Avoid
- Forgetting that DNA polymerase ONLY synthesises 5'→3' — this is why there's a lagging strand with Okazaki fragments.
- Confusing helicase (unwinds) with DNA polymerase (copies) — students often swap them.
- Describing replication as "conservative" or "dispersive" when it's actually SEMI-CONSERVATIVE (Meselson-Stahl).
- Missing the role of DNA ligase — it joins Okazaki fragments on the lagging strand.
- Drawing replication happening in just one direction — it proceeds bidirectionally from origins of replication.
Exam Strategy
HSC Module 5 DNA replication questions ask you to describe the process with enzyme specificity. Structure: (1) origin of replication, (2) helicase unwinds, (3) DNA polymerase synthesises new strands (leading continuous, lagging in Okazaki fragments), (4) ligase joins fragments, (5) proofreading by polymerase. The Meselson-Stahl experiment is a classic evidence question — be ready to interpret the banding patterns.
Sample Flashcards
Q1: Outline the process of DNA replication.
1) Helicase unwinds and separates the double helix. 2) DNA polymerase adds complementary nucleotides to each template strand (5' to 3'). 3) Leading strand is continuous; lagging strand is synthesised in Okazaki fragments. 4) Ligase joins fragments. Result: two identical DNA molecules (semi-conservative).
Sample Quiz Questions
Q1: DNA replication is described as semi-conservative because each new molecule contains one original strand and one new strand.
Answer: TRUE
Semi-conservative means each daughter DNA molecule has one parental (template) strand and one newly synthesised strand.
Revision Tip
Enzyme specificity is the main marking criterion — memorise the enzymes and their roles with Revizi flashcards, then practise labelling a replication fork diagram.
Related Concepts
Last updated: March 2026 · 1 flashcards · 1 quiz questions