translation elongation

Once the ribosome is assembled on the start codon with its first amino acid in place, the real work begins: adding the rest of the chain, one amino acid at a time, codon by codon, until the protein is complete. This steady, repeating heart of protein synthesis is elongation. Most of a protein's making is this one cycle, run over and over.

Each round of elongation is a simple three-beat rhythm. First, a charged tRNA whose anticodon matches the next codon enters the empty A site, escorted by an elongation factor that uses GTP to deliver it and to give the ribosome a moment to check the pairing. Second, the ribosome forms a peptide bond, transferring the entire growing chain from the tRNA in the P site onto the new amino acid in the A site — so the chain grows by exactly one residue. Third, the ribosome translocates: it ratchets forward by precisely one codon, pushing the now-empty tRNA toward the E site and shifting the chain-bearing tRNA into the P site, which opens the A site for the next codon. Then it all repeats. Each cycle costs the cell energy in the form of GTP, the price of speed and accuracy.

Elongation is astonishingly fast and faithful — a bacterial ribosome adds roughly fifteen to twenty amino acids per second, with an error rate around one in a few thousand. That accuracy is not free; it comes from a built-in pause during which mismatched tRNAs are given a chance to fall out before the bond forms, a kind of kinetic proofreading that trades a little extra GTP for far fewer mistakes. Because bacterial elongation factors and the bacterial ribosome differ from ours, several antibiotics (such as the aminoglycosides and the macrolides) work precisely by sabotaging bacterial elongation.