Translation

Translation builds a protein chain on an mRNA bound to a ribosome, in four steps: charging of tRNA, initiation, elongation and termination.

Charging. Each tRNA has an anticodon that pairs with a codon on the mRNA: for example, anticodon GAG pairs with codon CUC (glutamic acid). Fewer than 61 tRNAs are needed, because the pairing at the third base of a codon is loose: this is wobble. Enzymes called aminoacyl-tRNA synthetases (20 of them, one per amino acid) use ATP to attach each amino acid to the CCA end of its own tRNA.

Initiation. In bacteria, the small (30S) ribosome subunit binds a sequence on the mRNA called the Shine-Dalgarno sequence, which places the start codon AUG in the ribosome's P site, carrying the first tRNA with fMet (formyl-methionine). (Eukaryotes use plain Met-tRNA and scan from the cap to find AUG.) The large (50S) subunit then joins, with the help of initiation factors and GTP. The ribosome has three sites: A, P and E.

Elongation. A charged tRNA enters the A site (A for accept). The enzyme peptidyl transferase joins its amino acid to the growing chain held in the P site. The ribosome then moves along three bases (translocation), and the now-empty tRNA leaves from the E site (E for exit). This uses elongation factors and GTP.

Termination. When a stop codon reaches the A site, no tRNA matches it. Instead release factors recognise it and free the finished chain. Several ribosomes can read one mRNA at the same time (polyribosomes), and the new protein may be modified afterwards.