Chapter 6: Gene Expression: Translation and Protein Synthesis

Gene Expression: Translation and Protein Synthesis begins by defining the chemical composition of proteins, detailing how twenty standard amino acids—characterized by central carbons, amino groups, carboxyl groups, and distinct side chains—link via peptide bonds to form polypeptides that fold into complex primary, secondary, tertiary, and quaternary structures held together by hydrogen bonds and other molecular interactions. The text explores the nature of the genetic code, establishing it as a triplet, nonoverlapping, comma-free, and degenerate system where 64 codons specify amino acids, a discovery elucidated through frameshift mutation experiments by Crick and Brenner and synthetic mRNA assays by Nirenberg and Khorana. Key features such as the wobble hypothesis, which explains how a single tRNA can recognize multiple synonymous codons due to flexible pairing at the third position, and the near-universality of the code across organisms are emphasized. The discussion then shifts to the molecular machinery of translation, specifically the cloverleaf structure and function of transfer RNA (tRNA) with its anticodon loops and specific amino acid attachment sites charged by aminoacyl-tRNA synthetases, as well as the composition of ribosomes (70S in bacteria and 80S in eukaryotes) composed of ribosomal RNA and proteins. The mechanism of protein synthesis is dissected into three distinct stages: initiation, elongation, and termination. Initiation differences are highlighted by comparing the bacterial Shine-Dalgarno sequence interaction with 16S rRNA against the eukaryotic scanning model involving the 5-prime cap, eukaryotic initiation factors, and the Kozak sequence. The elongation phase is described in detail, involving the binding of charged tRNAs to the A site, peptide bond formation catalyzed by the ribozyme activity of peptidyl transferase within the large ribosomal subunit, and translocation powered by GTP hydrolysis and elongation factors like EF-Tu and EF-G. Termination is explained through the recognition of nonsense or stop codons by release factors and the subsequent disassembly of the ribosome complex. Finally, the chapter covers protein sorting and cellular trafficking, explaining the signal hypothesis where N-terminal signal sequences direct nascent polypeptides to the endoplasmic reticulum via signal recognition particles (SRP) for secretion or organelle placement, ensuring proteins reach their correct cellular destinations.