Chapter 12: Biochemical Signaling: Signal Transduction, Receptors, and the Cell Cycle

Section 12.1 introduces the different types of DNA repair pathways: direct repair, base-excision repair (BER), nucleotide-excision repair (NER), mismatch repair (MMR), and double-strand break repair. Each pathway is discussed with specific enzymes and molecular steps. Direct repair includes the reversal of alkylation by methyltransferases. BER corrects small base lesions using DNA glycosylases, AP endonucleases, DNA polymerase, and ligase. NER removes bulky lesions like thymine dimers via helicase and endonuclease action. MMR corrects replication mismatches using MutS, MutL, and MutH in bacteria. Double-strand breaks are repaired via homologous recombination (HR) or nonhomologous end joining (NHEJ), with HR employing RecA in prokaryotes or Rad51 in eukaryotes. Section 12.2 discusses recombination, highlighting homologous recombination during meiosis and the Holliday junction model. The process of strand invasion, branch migration, and resolution is presented in detail, emphasizing the role of recombinase enzymes. Site-specific recombination is also covered, including its role in phage genome integration and immune system gene rearrangement (e.g., V(D)J recombination in antibodies). Section 12.3 covers transposition, including DNA transposons and retrotransposons, and their evolutionary and regulatory significance. It explains transposase function and the cut-and-paste vs copy-and-paste mechanisms. Section 12.4 addresses the regulation of DNA repair and recombination, the importance of cell cycle checkpoints, and the role of p53 and other proteins in damage recognition and apoptosis induction. The chapter ends by tying DNA repair and recombination to cancer, inherited disease, and therapeutic strategies, including CRISPR-Cas9 genome editing. Altogether, this chapter underscores the dynamic yet tightly controlled nature of genomic maintenance in all living organisms.