Chapter 7: Genomic DNA Replication, Repair, & Rearrangements

Genomic DNA Replication, Repair, & Rearrangements begins by detailing the semiconservative mechanism of DNA replication, driven by DNA polymerases that synthesize new strands in the 5-prime to 3-prime direction, and explains the necessity of primase-synthesized RNA primers for initiating synthesis on the continuous leading strand and the discontinuous lagging strand (which forms Okazaki fragments). The summary differentiates between the enzymatic machinery in prokaryotes, such as Polymerase III, and eukaryotes, including Polymerases alpha, delta, and epsilon, while highlighting the critical functions of accessory proteins like sliding clamps (PCNA), clamp loaders (RFC), helicases, and single-stranded DNA-binding proteins (RPA). It also addresses the topological challenges solved by topoisomerases and the unique role of telomerase, a reverse transcriptase that maintains chromosome ends to prevent genomic instability and cellular senescence. The discussion progresses to DNA repair mechanisms that counteract environmental damage and replication errors, categorizing them into direct reversal (photoreactivation), base-excision repair utilizing DNA glycosylases, nucleotide-excision repair (noting its relevance to Xeroderma Pigmentosum), and mismatch repair, defects in which are linked to hereditary colon cancers like HNPCC. Furthermore, the chapter explores how cells handle severe lesions through error-prone translesion synthesis or repair potentially lethal double-strand breaks via nonhomologous end joining and homologous recombination. Finally, the text covers programmed DNA rearrangements and gene amplification, illustrating how mechanisms like V(D)J recombination and class switch recombination generate the vast diversity of antibodies and T cell receptors in the immune system, and how gene amplification can function as a developmental tool or a driver of oncogene overexpression in cancer.