ribosome structure

Meaning: The small subunit binds mRNA and checks codon-anticodon pairing. The large subunit catalyzes peptide bond formation. Each subunit is rRNA plus proteins, and the rRNA does the catalysis, so the ribosome is a ribozyme. Sizes are given in Svedberg units (S), which measure sedimentation rate: prokaryotes: 70S = 30S + 50S eukaryotes: 80S = 40S + 60S Three tRNA binding sites lie between the subunits. The A site (aminoacyl) receives the incoming charged tRNA, the P site (peptidyl) holds the tRNA carrying the growing chain, and the E site (exit) releases the empty tRNA. The ribosome moves along mRNA 5′ → 3′, so tRNAs shift A → P → E. In eukaryotes, rRNA is made and the subunits assembled in the nucleolus. Free ribosomes make cytosolic proteins. Ribosomes on the rough ER make proteins that are secreted, put in membranes or sent to lysosomes.

On the MCAT: Expect questions that compare prokaryotic and eukaryotic ribosomes, since the structural differences explain why some agents inhibit bacterial translation without blocking eukaryotic translation. Other questions ask which site holds the growing chain, or what happens if a site is blocked.

Common confusion: Svedberg values are not additive. 30S + 50S gives 70S, not 80S, because S depends on both mass and shape. Also, a ribosome has no membrane and is not an organelle in the membrane-bound sense.