DNA Structure & Replication

How cells accurately copy their genetic blueprint

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

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

  • 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.

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

Quiz Yourself

1. Which enzyme unwinds the DNA double helix?

2. What removes RNA primers on the lagging strand?

3. Leading strand is synthesized: