Chapter 10: DNA Replication & Chromosome Duplication

The core principle established is semiconservative replication, where each parental strand serves as a conserved template for synthesizing a new complementary progeny strand, a model experimentally validated in E. coli by Meselson and Stahl through equilibrium density-gradient centrifugation. Replication initiates at specified origins of replication, such as oriC in prokaryotes, and generally proceeds bidirectionally. Due to the strict limitation that DNA polymerases can only catalyze synthesis in the 5' to 3' direction, DNA synthesis at the replication fork is asymmetric: the leading strand is synthesized continuously, while the lagging strand is built discontinuously from short pieces known as Okazaki fragments. New DNA chains require initial RNA primers synthesized by DNA primase to provide a free 3'-OH group. The complex machinery, known collectively as the replisome, employs various accessory proteins, including DNA helicases for unwinding the double helix, SSB proteins for stabilizing the single-stranded templates, and DNA topoisomerases (like DNA gyrase) for relieving positive supercoiling stress ahead of the fork. In E. coli, DNA polymerase III is the primary replicase, while DNA polymerase I is essential for removing RNA primers and filling the resulting gaps, which are then sealed by DNA ligase. High accuracy is maintained through enzymatic proofreading, primarily carried out by the 3' → 5' exonuclease activity built into many DNA polymerases. Eukaryotic replication features unique elements, including confinement to the S phase and the use of multiple replicons. Eukaryotic replisomes utilize multiple specialized polymerases: Pol α (for priming), Pol δ (lagging strand synthesis, aided by the PCNA sliding clamp), and Pol ε (leading strand synthesis). Finally, the chapter addresses the end-replication problem of linear chromosomes, solved by the specialized ribonucleoprotein enzyme telomerase, which extends the G-rich terminal sequences (like TTAGGG) to prevent cumulative shortening, a process whose absence is correlated with cellular senescence.