DNA Replication & Repair

The Replication Machine

• Replication is semiconservative (Meselson-Stahl) and bidirectional; polymerases synthesize only $5' \to 3'$ from a free 3'-OH, so primase must lay RNA primers.
• Enzyme order: helicase unwinds, topoisomerase (gyrase) relieves supercoils ahead, SSB proteins hold strands apart, DNA Pol III extends, DNA Pol I replaces primers, ligase seals nicks.
• The leading strand runs continuously toward the fork; the lagging strand builds Okazaki fragments away from it, each with its own primer.

Telomeres and Fidelity

• Linear chromosomes shorten each division (end replication problem); telomerase, a reverse transcriptase with an internal RNA template, extends TTAGGG repeats in stem, germ, and cancer cells.
• Accuracy improves stepwise: $\text{Pol III alone} \sim 10^{-5} \to \text{proofreading} \sim 10^{-7} \to \text{mismatch repair} \sim 10^{-9}$

Repair Pathways

• Mismatch repair fixes post-replication errors (defect: Lynch syndrome). Base excision repair handles deaminated or oxidized bases. Nucleotide excision repair removes bulky lesions like UV thymine dimers (defect: xeroderma pigmentosum). Double-strand breaks use accurate homologous recombination (BRCA1/2, needs a sister chromatid) or error-prone NHEJ.

Key Takeaways

• Memorize the enzyme sequence at the fork and why the lagging strand is discontinuous.
• Telomerase activity separates immortal cells from aging somatic cells.
• Match each repair pathway to its lesion and its disease.
• HR is template-guided and faithful; NHEJ just glues ends.