Chapter 22: Protein Synthesis

The genetic code serves as the fundamental translation system, consisting of sixty-four three-letter codons that specify the twenty standard amino acids, with most amino acids encoded by multiple synonymous codons. The code begins at an initiation codon and terminates at designated stop codons, remaining largely universal across all organisms. Transfer RNA molecules function as adapters, recognizing mRNA codons through anticodon pairing while carrying their corresponding amino acids, with wobble pairing at the third codon position allowing a single tRNA to decode multiple synonymous codons. Aminoacyl-tRNA synthetases catalyze the precise attachment of amino acids to their cognate tRNAs using ATP energy, incorporating proofreading mechanisms to maintain translation accuracy. Ribosomes, massive RNA-protein complexes composed of distinct subunits, catalyze peptide bond formation between amino acids at three critical tRNA binding sites. Translation unfolds through three coordinated stages: initiation, where translation machinery assembles on the mRNA start site with assistance from regulatory proteins; elongation, a repetitive cycle where elongation factors deliver aminoacyl-tRNAs, peptidyl transferase catalyzes bond formation, and translocase shifts the mRNA by one codon; and termination, when release factors recognize stop codons and hydrolyze the completed polypeptide chain. The process is energetically expensive, requiring four high-energy phosphate bonds per amino acid incorporated. Regulation primarily occurs at initiation through mechanisms such as feedback inhibition in prokaryotes and heme-responsive kinase signaling in eukaryotes. Following synthesis, polypeptides undergo post-translational modifications and directed targeting, with secreted and membrane proteins containing signal peptides recognized by signal recognition particles that direct ribosomes to the endoplasmic reticulum for membrane insertion.