Chapter 11: DNA Replication

DNA replication represents the fundamental process by which cells accurately duplicate their genetic material before division, ensuring faithful transmission of hereditary information across generations. This chapter examines both the theoretical foundations and molecular mechanisms underlying DNA synthesis, beginning with the semiconservative replication model proposed by Watson and Crick. The Meselson-Stahl experiment provides the definitive experimental evidence that each daughter DNA molecule consists of one original parental strand paired with one newly synthesized strand, establishing this model as correct. The chapter then contrasts bacterial and eukaryotic replication systems, highlighting their distinct organizational strategies and enzymatic requirements. In prokaryotes like E. coli, replication initiates at a single origin where DnaA proteins facilitate strand separation, followed by helicase unwinding the double helix and single-strand binding proteins stabilizing the exposed strands. Topoisomerase relieves the tension created by unwinding, while primase generates short RNA primers that serve as starting points for DNA synthesis. DNA polymerase III extends continuously along the leading strand while simultaneously constructing the lagging strand through discontinuous Okazaki fragments, which are subsequently connected by DNA polymerase I and DNA ligase. Eukaryotic replication introduces greater complexity through multiple replication origins, requiring specialized regulatory proteins and a broader array of polymerases to manage the larger genome efficiently. Pre-replication complexes assemble during G1 phase and activate during S phase, with DNA polymerases alpha, delta, and epsilon orchestrating synthesis while sliding clamp proteins enhance enzyme processivity. The chapter addresses the end-replication problem inherent to linear chromosomes and explains how telomerase, a reverse transcriptase enzyme containing an RNA template, maintains telomeric sequences and prevents progressive chromosome shortening. Regulatory mechanisms, quality-control checkpoints, and fidelity processes including polymerase proofreading are explored as essential safeguards against mutations. The discussion includes experimental methodologies such as pulse-chase labeling and temperature-sensitive mutant analysis that have revealed replication dynamics, emphasizing that precise DNA replication is critical not only for hereditary continuity but also for preventing genomic instability that could lead to malignancy or developmental pathology.