describe the semi-conservative replication of DNA during the S phase of the cell cycle, including: the roles of DNA polymerase and DNA ligase (knowledge of other enzymes in DNA replication in cells and different types of DNA polymerase is not expecte

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Structure of Nucleic Acids and DNA Replication

Structure of Nucleic Acids and Replication of DNA

Key Objective

Describe the semi‑conservative replication of DNA during the S phase of the cell cycle, focusing on:

  • The role of DNA polymerase

  • The role of DNA ligase

  • Differences between leading‑strand and lagging‑strand synthesis

Overview of DNA Replication

During the S (synthesis) phase, each chromosome is duplicated so that two identical daughter chromosomes are produced. Replication is semi‑conservative: each new DNA molecule contains one original (parental) strand and one newly synthesised strand.

Steps of Semi‑Conservative Replication

  1. Initiation at the origin of replication

    • Specific DNA sequences (origins) are recognised, and the double helix is locally unwound.

    • Helicase (not required for this syllabus) creates a replication fork.

  2. Stabilisation of single strands

    • Single‑strand binding proteins prevent re‑annealing of the separated strands.

  3. Synthesis of new strands

    • DNA polymerase adds nucleotides only in the 5′→3′ direction.

    • Because the two template strands run antiparallel, synthesis occurs continuously on one strand (leading) and discontinuously on the other (lagging).

  4. Removal of RNA primers and joining of fragments

    • RNA primers are replaced with DNA nucleotides.

    • DNA ligase forms phosphodiester bonds between adjacent DNA fragments (Okazaki fragments) on the lagging strand.

  5. Proofreading

    • DNA polymerase possesses 3′→5′ exonuclease activity that removes incorrectly paired nucleotides.

Roles of Key Enzymes

  • DNA polymerase

    • Catalyses the addition of deoxyribonucleotides to the 3′‑OH end of the growing strand.

    • Operates only in the 5′→3′ direction, which dictates the need for leading and lagging strand synthesis.

    • Proofreads newly added nucleotides, reducing the error rate.

  • DNA ligase

    • Forms phosphodiester bonds between adjacent DNA fragments.

    • Essential for sealing the nicks between Okazaki fragments on the lagging strand, producing a continuous DNA strand.

Leading vs. Lagging Strand Synthesis

FeatureLeading StrandLagging Strand
Direction of synthesis relative to fork movementContinuous synthesis in the same direction as the replication fork opens (5′→3′)Discontinuous synthesis opposite to fork movement; formed as short fragments
Type of fragments producedSingle, continuous strandOkazaki fragments (≈100–200 nt in eukaryotes)
Requirement for RNA primersOne primer at the originMultiple primers, one for each Okazaki fragment
Enzyme activity needed after synthesisMinimal – polymerase continues adding nucleotidesDNA ligase required to join fragments
Overall speed of synthesisRelatively faster, uninterruptedAppears slower due to repeated priming and ligation steps

Illustrative Diagram

Suggested diagram: Replication fork showing leading‑strand continuous synthesis, lagging‑strand Okazaki fragments, positions of DNA polymerase and DNA ligase.

Key Points to Remember

  • DNA replication is semi‑conservative – each daughter molecule contains one old and one new strand.

  • DNA polymerase can only add nucleotides to the 3′‑OH end; therefore synthesis proceeds 5′→3′.

  • The antiparallel nature of DNA forces the cell to use two different strategies (leading vs. lagging) to copy both strands.

  • DNA ligase is essential for creating a continuous strand on the lagging side by joining Okazaki fragments.

  • Proofreading by DNA polymerase ensures high fidelity of the replicated genome.