Chapter 15: Translation: Synthesis of Proteins

The code exhibits degeneracy, meaning multiple codons encode the same amino acid, yet remains unambiguous in its assignments. Crick's wobble hypothesis explains how this redundancy is managed through flexible base pairing at the third codon position, allowing fewer transfer RNA molecules to recognize the full complement of codons. Transfer RNA molecules function as molecular adapters, with their anticodon regions binding to complementary mRNA codons while their amino acid attachment sites carry specific amino acids. Aminoacyl-transfer RNA synthetases catalyze the charging of transfer RNAs with their cognate amino acids, ensuring translational fidelity through precise substrate recognition. The chapter details three distinct stages of translation: initiation, which involves formation of the translation initiation complex through recognition of start sequences and recruitment of initiation factors and the initiator transfer RNA; elongation, during which amino acids are sequentially added through the action of elongation factors and the catalytic peptidyltransferase activity of ribosomal RNA; and termination, which occurs when release factors recognize stop codons and hydrolyze the nascent polypeptide from the ribosome. Eukaryotic and prokaryotic translation mechanisms differ in their initiation sequences, initiator transfer RNAs, and factor requirements, with eukaryotes recognizing the Kozak sequence and prokaryotes using the Shine-Dalgarno sequence. Protein synthesis is energetically expensive, consuming four high-energy phosphate equivalents per peptide bond formed. Following translation, proteins undergo extensive posttranslational modifications including folding mediated by molecular chaperones, formation of disulfide bonds, proteolytic processing, glycosylation, phosphorylation, and lipidation that collectively determine protein function and localization. Targeting sequences direct proteins through the endoplasmic reticulum and Golgi apparatus to their final destinations, whether compartmentalized within organelles or secreted. The chapter integrates clinical cases demonstrating translational disorders and therapeutic interventions, including how mutations cause sickle cell disease, thalassemia, and progeria; how defective lysosomal targeting in I-cell disease reflects impaired posttranslational marking; and how antibiotics and toxins selectively inhibit bacterial or eukaryotic protein synthesis by targeting ribosomal function.