translation elongation
/ tranz-LAY-shun ee-long-GAY-shun /
Once the ribosome is assembled and aimed at the start of the message, the real assembly-line work begins: adding amino acids one by one, in the order the codons dictate, to build the growing protein chain. That repetitive, cyclical phase is called elongation. If initiation is switching the machine on, elongation is the machine actually stamping out the product, link by link.
Elongation is a three-step cycle the ribosome repeats for every codon. First, a transfer RNA whose anticodon matches the next codon enters the ribosome's A site, delivering its amino acid. Second, the ribosome catalyzes a peptide bond, transferring the whole growing chain onto that newly arrived amino acid — so the chain lengthens by one. Third comes translocation: the ribosome ratchets forward exactly three bases along the messenger RNA, shifting the tRNAs from the A and P sites to the P and E sites, and ejecting the now-empty tRNA from the E site. The A site is open again, and the cycle repeats. Helper proteins called elongation factors, powered by GTP, drive the tRNA delivery and the forward ratcheting.
Elongation matters because it is where the protein's sequence is physically written, and it is impressively fast and accurate — our ribosomes add a few amino acids per second with very few mistakes. The ribosome's three tRNA sites (A for arrival/aminoacyl, P for the growing peptide, E for exit) and the peptide-bond chemistry are the heart of how a one-dimensional code becomes a real chain of amino acids. A common simplification to resist: elongation is not one smooth motion but a tightly choreographed loop of bind, bond, and step that runs over and over.
The antibiotic tetracycline works by blocking the ribosome's A site, so no new tRNA can dock and elongation stalls. Because bacterial and human ribosomes differ enough, it jams the bacteria's assembly line while largely sparing ours.
Several antibiotics work by jamming one step of the elongation cycle.
The peptide-bond-forming activity sits in the large subunit's RNA, not its protein — the ribosome is a ribozyme. So strictly speaking it is RNA, not protein, that catalyzes the central reaction of elongation.