Chapter 13: Bacterial Genome Replication & Gene Expression

Bacterial Genome Replication & Gene Expression explores the fundamental processes of bacterial genome replication and gene expression, starting with the historical foundation establishing DNA as the primary storage molecule for genetic information, supported by definitive studies such as the Griffith transformation experiments and the work of Hershey and Chase. The text outlines the intricate molecular structures of DNA, RNA, and proteins, detailing how deoxyribonucleotides form the antiparallel, complementary double helix that undergoes supercoiling, and how amino acids link via peptide bonds. DNA replication in bacteria is described as a semiconservative process initiating at a single origin of replication and proceeding bidirectionally through the activity of the replisome complex. Key enzymes include DNA polymerase III (for 5' to 3' synthesis and proofreading), helicases, DNA gyrase, and primase, which together manage the continuous leading strand and the discontinuous lagging strand composed of Okazaki fragments that are later joined by DNA polymerase I and DNA ligase. Shifting to gene expression, the chapter defines the structure of bacterial genes, including the promoter, coding region, and terminator, noting that related genes are often organized into operons that produce polycistronic mRNA. Transcription is catalyzed by RNA polymerase holoenzyme, guided by sigma factors to the promoter, and concludes via factor-independent or rho-dependent termination. The chapter decodes the genetic code, explaining its degeneracy and the role of the wobble hypothesis. Translation, the synthesis of polypeptide chains, occurs rapidly on ribosomes (often forming a polysome) after amino acids are activated and attached to tRNA by aminoacyl-tRNA synthetases. This three-stage process involves initiation using fMet-tRNA, elongation via transpeptidation (catalyzed by peptidyl transferase), and translocation across the A, P, and E sites of the ribosome, culminating in release at a stop codon. Finally, the chapter covers post-translational modifications, including assistance from molecular chaperones for proper protein folding and the mechanisms of protein translocation (Sec and Tat systems) and specialized secretion systems (Types I–VII) utilized by bacteria to move proteins across membranes and into the extracellular environment.