Describe the structure and function of ribosomes.

Ribosomes are ribonucleoprotein complexes that catalyze protein synthesis. Prokaryotic 70S ribosomes consist of 30S (16S rRNA + 21 proteins) and 50S (23S, 5S rRNA + 34 proteins) subunits. Eukaryotic 80S ribosomes have 40S and 60S subunits. The ribosome has three tRNA binding sites: A (aminoacyl, incoming), P (peptidyl, growing chain), and E (exit). The small subunit decodes mRNA, while the large subunit catalyzes peptide bond formation via its peptidyl transferase center, which is an RNA enzyme (ribozyme). Structural differences between bacterial and eukaryotic ribosomes enable antibiotic targeting.