the A, P, and E sites
Picture a small assembly line inside the ribosome with exactly three numbered stations. A transfer RNA enters at the first station, does its work at the second, and leaves from the third. These three stations are the A, P and E sites, and the orderly march of tRNAs through them is the rhythm of protein synthesis.
Each site has a clear job, and its name tells you what it does. The A site (aminoacyl site) is the arrivals gate: a new charged tRNA, carrying the next amino acid, enters here and its anticodon is checked against the mRNA codon. The P site (peptidyl site) holds the tRNA attached to the whole growing protein chain so far. When a correct tRNA has settled in the A site, the ribosome transfers the entire chain from the P-site tRNA onto the A-site amino acid, lengthening it by one. The ribosome then ratchets forward by exactly one codon (translocation), which slides the now-empty P-site tRNA into the E site (exit site), from which it is released, while the chain-bearing tRNA moves into the P site and a fresh codon opens up in the A site. The cycle repeats.
These three sites are why translation is fast, ordered and accurate rather than a chaotic scramble. The A site is also the proofreading checkpoint: an incorrectly paired tRNA tends to fall out before the chemistry commits, so most wrong tRNAs are rejected here at the cost of a little energy. And because antibiotics that gum up the A site (like tetracycline, which blocks tRNA entry) or freeze translocation can selectively halt bacterial ribosomes, this little three-station line is a heavily exploited drug target.
Snapshot mid-elongation: the P site holds a tRNA bearing the chain Met-Ala-Gly; a new Ser-tRNA has just docked in the A site reading the next codon UCA; the E site holds the spent tRNA from the previous round, about to be ejected.
tRNAs move A -> P -> E: enter, hold the chain, then exit, one codon per step.
The growing chain is always handed from the P-site tRNA onto the A-site amino acid, not the other way round — the chain moves forward onto the newcomer, then the whole assembly ratchets one codon along.