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 · 8 days ago

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

Structure of Nucleic Acids and Replication of DNA

Learning Objective

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

  • The role of DNA polymerase and DNA ligase (no detailed knowledge of other enzymes is required).

  • The differences between leading‑strand and lagging‑strand synthesis as a consequence of DNA polymerase adding nucleotides only in the 5′→3′ direction.

Key Concepts

Semi‑conservative Replication

During the S phase each double‑helix DNA molecule separates into two single strands. Each original (parental) strand serves as a template for the synthesis of a new complementary strand. After replication each daughter DNA molecule consists of one parental strand and one newly synthesised strand.

Directionality of DNA Synthesis

DNA polymerase can only add nucleotides to the 3′‑hydroxyl end of a growing strand. Consequently synthesis proceeds in the 5′→3′ direction, i.e. new nucleotides are added to the 3′ end of the strand.

Enzymes Involved

  • DNA polymerase – catalyses the addition of deoxyribonucleotides to the 3′ end of the nascent strand, using the parental strand as a template.

  • DNA ligase – joins adjacent Okazaki fragments on the lagging strand by forming phosphodiester bonds between the 3′‑OH and 5′‑phosphate groups.

Leading vs. Lagging Strand Synthesis

Because the two parental strands are antiparallel, one can be copied continuously (leading strand) while the other must be copied discontinuously (lagging strand).

FeatureLeading StrandLagging Strand
Direction of synthesis relative to replication forkSame direction as fork movement (continuous)Opposite direction to fork movement (discontinuous)
Mode of synthesisContinuous synthesis by a single DNA polymeraseSeries of short fragments (Okazaki fragments) synthesised
Primer requirementSingle RNA primer at originNew RNA primer for each Okazaki fragment
Enzyme that joins fragmentsNot required (continuous strand)DNA ligase joins adjacent Okazaki fragments
Resulting strand after ligationIntact, uninterrupted phosphodiester backboneIntact strand formed after ligase seals nicks

Step‑by‑Step Overview of Replication

  1. Helicase unwinds the double helix, creating a replication fork.

  2. Single‑strand binding proteins stabilise the separated strands.

  3. RNA primase synthesises a short RNA primer on each template strand.

  4. DNA polymerase adds nucleotides to the 3′ end of each primer:

    • On the leading strand synthesis proceeds continuously toward the fork.

    • On the lagging strand synthesis proceeds away from the fork, forming Okazaki fragments.

  5. DNA polymerase replaces RNA primers with DNA nucleotides.

  6. DNA ligase joins the Okazaki fragments on the lagging strand, creating a continuous phosphodiester backbone.

  7. Topoisomerase relieves supercoiling ahead of the fork.

Key Equation (DNA Base Pairing)

\$\text{A} \leftrightarrow \text{T}, \qquad \text{G} \leftrightarrow \text{C}\$

Suggested diagram: A schematic of a replication fork showing leading‑strand continuous synthesis, lagging‑strand Okazaki fragments, and the positions of DNA polymerase and DNA ligase.

Summary

DNA replication is semi‑conservative: each daughter molecule contains one original strand and one newly synthesised strand. DNA polymerase adds nucleotides only in the 5′→3′ direction, which necessitates continuous synthesis on the leading strand and discontinuous synthesis (Okazaki fragments) on the lagging strand. DNA ligase is essential for sealing the nicks between fragments on the lagging strand, producing a complete, stable double helix ready for cell division.