The ribosome is a ~2.5 MDa ribonucleoprotein complex that synthesizes proteins at 5–20 amino acids per second. It has three sites: A (aminoacyl — incoming tRNA), P (peptidyl — growing chain), E (exit — leaving tRNA). The 30S subunit decodes mRNA codons by Watson-Crick base pairing; the 50S catalyzes peptide bond formation via peptidyl transferase activity (the active site is RNA, not protein — a ribozyme). The GTPase EF-Tu delivers aminoacyl-tRNA to the A site; EF-G drives translocation after peptide transfer. Fidelity is extraordinary: error rate ~1 in 10,000 codons, enhanced by kinetic proofreading (Hopfield 1974).