Chapter 17: DNA Replication, Repair & Recombination

DNA Replication, Repair & Recombination begins by establishing the semiconservative model of DNA replication, a landmark concept proven by the Meselson-Stahl experiment, which reveals that each new DNA molecule consists of one original parental strand and one newly synthesized strand. The orchestration of this process involves a suite of specialized proteins: helicases to unwind the double helix, single-stranded binding proteins to stabilize the open strands, and topoisomerases like DNA gyrase to prevent mechanical tension and tangling ahead of the replication fork. Because DNA polymerases can only add nucleotides in a specific direction, the replication fork is inherently asymmetrical, featuring a leading strand synthesized continuously and a lagging strand created in short segments known as Okazaki fragments. These fragments are initiated by RNA primers and eventually stitched together by DNA ligase. In eukaryotes, the challenge of replicating the very ends of linear chromosomes is addressed by telomeres and the enzyme telomerase, which prevents the loss of vital genetic information during successive cell divisions. To ensure the integrity of the genome, cells possess sophisticated proofreading and repair systems that correct errors occurring during replication or resulting from environmental damage. Spontaneous chemical changes such as depurination and deamination, along with external mutagens like ultraviolet light—which triggers the formation of pyrimidine dimers—are countered by pathways including base excision repair, nucleotide excision repair, and mismatch repair. Mutations in these systems can lead to severe clinical conditions, such as xeroderma pigmentosum, where individuals are dangerously sensitive to sunlight. For the most critical threats, like double-strand breaks, cells utilize either the quick but error-prone nonhomologous end-joining or the high-fidelity homologous recombination pathway, the latter of which involves complex structural intermediates called Holliday junctions. The chapter concludes by exploring the dynamic nature of the genome through transposable elements, or "jumping genes," which use various mechanisms to move across different chromosomal locations, thereby driving genetic diversity and evolutionary change.