- Helicase: unwinds helix at the replication fork.
- Primase: lays down RNA primers.
- DNA Polymerase: adds nucleotides 5′→3′ with proofreading.
- Exonuclease: removes RNA primers.
- DNA Ligase: seals nicks between Okazaki fragments.
Chapter Summary
DNA replication is semiconservative: each daughter duplex contains one parental and one new strand. Key steps involve unwinding, primer synthesis, elongation, primer removal, and ligation. Fidelity is ensured by proofreading and repair.
DNA Structure
Double helix of two antiparallel strands held by A–T (2 H-bonds) and C–G (3 H-bonds) base pairing.
Key Enzymes & Mechanisms
Proofreading & Repair
DNA Polymerase III (and δ in eukaryotes) has 3′→5′ exonuclease activity to remove mispaired bases. Mismatch repair enzymes fix errors post-replication. Defects in repair lead to mutations and disease (e.g., cancer).
Clinical Correlations
- Xeroderma pigmentosum: defective nucleotide excision repair → UV-induced DNA damage accumulates.
- Chemotherapy: drugs (e.g., aphidicolin) inhibit DNA polymerases to block replication in cancer cells.
Key Terms
- Semiconservative: each daughter helix has one old and one new strand.
- Okazaki Fragments: short lagging-strand segments.
- Proofreading: 3′→5′ exonuclease activity.
- Replication Fork: Y-shaped junction where unwinding occurs.
Quiz Yourself
1. Which enzyme unwinds the DNA double helix?
2. What removes RNA primers on the lagging strand?
3. Leading strand is synthesized: