Chapter 28: Regulation of Gene Expression: Operons, Transcription Factors, and Epigenetic Control

Section 28.1 details the overall process of DNA synthesis, beginning with origin recognition and helicase-driven strand separation. Single-stranded DNA is stabilized by SSB proteins, and topoisomerases relieve supercoiling ahead of the replication fork. DNA polymerases synthesize the new strands in the 5’ to 3’ direction, requiring an RNA primer laid down by primase. The chapter explains the leading and lagging strand synthesis, emphasizing Okazaki fragment formation and processing. DNA polymerase III (in prokaryotes) and DNA polymerases α, δ, and ε (in eukaryotes) carry out bulk synthesis. Sliding clamp proteins (e.g., β-clamp or PCNA) and clamp loaders enhance processivity. DNA ligase seals nicks after primer removal, ensuring a continuous DNA strand. The chapter also describes the role of telomerase in extending the ends of linear chromosomes, particularly in stem cells and cancer. Section 28.2 transitions to DNA repair mechanisms essential for maintaining genome integrity. The chapter highlights direct repair (e.g., photolyase in bacteria), base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), and double-strand break repair (DSBR) via homologous recombination or nonhomologous end joining (NHEJ). Key enzymes such as DNA glycosylases, AP endonuclease, and recombinases (like RecA and Rad51) are discussed in context. The chapter integrates molecular details with biological implications, such as the causes and consequences of replication errors, the importance of proofreading and 3'→5' exonuclease activity, and the link between defective repair pathways and diseases like xeroderma pigmentosum, Lynch syndrome, and various cancers. DNA replication and repair are presented as tightly regulated, highly coordinated, and essential for heredity, genome stability, and cell viability.