Chapter 6: Molecular Information Flow and Protein Processing

Molecular Information Flow and Protein Processing explains how genetic information flows from DNA to RNA to protein and how newly synthesized proteins are processed, folded, and transported within microbial cells. It begins with the central dogma of molecular biology, describing how DNA stores genetic information that is transcribed into RNA and ultimately translated into proteins that perform cellular functions. The structure of nucleic acids is examined through the double-helical organization of DNA, complementary base pairing, antiparallel strands, and the role of phosphodiester bonds in forming the nucleic acid backbone. The chapter details DNA replication as a semiconservative process beginning at the origin of replication, involving enzymes such as DNA helicase, primase, DNA polymerase, and DNA ligase within the replisome complex. Continuous synthesis of the leading strand and discontinuous synthesis of the lagging strand through Okazaki fragments illustrate how cells duplicate their genomes with high fidelity through proofreading mechanisms. The process of transcription is then explored, where RNA polymerase binds promoter regions to synthesize messenger RNA, transfer RNA, and ribosomal RNA, with bacterial operons producing polycistronic mRNA and eukaryotic transcripts undergoing RNA processing such as splicing, capping, and polyadenylation. The chapter continues with translation, where ribosomes interpret the genetic code by reading mRNA codons and pairing them with tRNA anticodons carrying amino acids to form polypeptide chains through peptide bond formation. The stages of initiation, elongation, and termination are described along with the degeneracy of the genetic code, start and stop codons, ribosome binding sites, and the formation of polysomes. Finally, the chapter explores protein maturation and cellular targeting, including folding assisted by chaperone proteins, post translational modifications, and the transport of proteins across cellular membranes through secretion systems such as the Sec pathway, Tat pathway, and specialized gram negative bacterial secretion systems that deliver proteins to the periplasm, the extracellular environment, or directly into host cells.