ACT SSC Biology — Unit 3
DNA Replication — ACT SSC Biology — Unit 3 Flashcards & Quiz
DNA replication belongs to Unit 3 Heredity and Continuity of Life in the BSSS Biology A/T course (from 2023), under 'DNA, genes and the continuity of life'. The course states that continuity of life requires genetic material to be copied and passed on through binary fission, mitosis, meiosis and fertilisation, and that DNA's structure (complementary base pairing and weak bonds between strands) allows replication. The enzyme detail on this page (helicase, DNA polymerase, ligase) is the standard explanation of how that copying happens, and the Meselson–Stahl experiment is the classic evidence that it is semi-conservative.
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
ACT SSC Biology is assessed through tasks set and marked by your college, so formats vary, but replication tasks commonly ask you to describe the process or interpret a diagram of a replication fork. Structure: (1) link DNA's base-pairing structure to its ability to be copied, (2) helicase unwinds the helix at an origin of replication, (3) DNA polymerase adds complementary nucleotides 5'→3', continuously on the leading strand and in Okazaki fragments on the lagging strand, (4) ligase joins the fragments, (5) proofreading keeps errors low. Label diagrams clearly.
Sample Flashcards
Q1: Why is DNA replication described as semi-conservative?
Each new DNA molecule contains one original (parental) strand and one newly synthesised strand, because the two parental strands separate and each acts as a template. Meselson and Stahl (1958) supported this by growing bacteria in heavy nitrogen (¹⁵N) and then light nitrogen (¹⁴N). After one generation all DNA had intermediate density, ruling out conservative replication; after two, half was intermediate and half light.
Q2: How does the structure of DNA allow it to be replicated?
DNA is a double helix of two antiparallel nucleotide strands held together by weak hydrogen bonds between complementary bases: adenine with thymine, and cytosine with guanine. Because these bonds are weak, the strands can be separated without breaking the sugar–phosphate backbones. Because pairing is complementary, each strand specifies the base sequence of its partner, so it can serve as a template for a new strand.
Q3: What are the roles of helicase and DNA polymerase in replication?
Helicase unwinds the double helix at the replication fork, breaking the hydrogen bonds between bases so the strands separate. Primase lays down a short RNA primer to start each new strand. DNA polymerase then adds free nucleotides complementary to the template, joining each to the 3' end of the growing strand, so new DNA is always built 5' to 3'. Polymerase also proofreads, removing mismatched nucleotides.
Q4: Why is the lagging strand made as Okazaki fragments?
The two template strands are antiparallel, but DNA polymerase can only add nucleotides to a 3' end, building new DNA 5' to 3'. On the leading strand this direction runs towards the replication fork, so synthesis is continuous. On the lagging strand it runs away from the fork, so DNA is made in short pieces called Okazaki fragments, each started from a new primer, which DNA ligase later joins.
Q5: What does DNA ligase do?
DNA ligase forms the phosphodiester bonds that join adjacent DNA fragments into one continuous strand. During replication it seals the gaps between Okazaki fragments on the lagging strand after the RNA primers have been removed and replaced with DNA. Without ligase the lagging strand would stay in fragments. Ligase is also used in gene technology to join DNA fragments into plasmids.
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.
Related Concepts
Last updated: 9 October 2026 · 6 sample flashcards · 1 sample quiz questions