In prokaryotic cells, translation is initiated by formation of an initiation complex consisting of the 30S ribosomal subunit, formal methionineal tRNA, and messenger RNA. The 50S ribosomal subunit is the first ribosomal subunit to be initiated by the 30S ribosomal subunit. subunit then joins the complex.
Proteins called initiation factors are also involved but are not shown. The 70S ribosome has two sites to which transfer RNA carrying amino acids can bind. One One is called the peptidyl or P site and the other is called the acceptor or A site.
There is also a third site called the exit or E site where transfer RNAs are released. The initiating transfer RNA carrying formal methionine binds to the P site. A transfer RNA that recognizes the next codon and carries the second amino acid then moves into the A site. The formal methionine carried by the transfer RNA in the P site is then joined to the amino acid carried by the transfer RNA that just entered the A site by a peptide bond. The ribosome now advances a distance.
of one codon and the transfer RNA that carried the formal methionine is released at the E site. A transfer RNA carrying the next amino acid now moves into the A site where the anticodon on the transfer RNA matches the codon on the messenger RNA. The ribosome shifts down by a distance of one codon. As the shift occurs, the two amino acids on the transfer RNA in the P site are transferred to the new amino acid and the second transfer RNA is released from the E site. The ribosome continues to move along the messenger RNA and new amino acids are added to the growing polypeptide chain.
Elongation of the polypeptide is terminated when a stop codon moves into the A site. A stop codon does not specify an amino acid and does not have a corresponding transfer RNA. The ribosome dissociates into the 30S and 50S subunits and the messenger RNA and protein are released.