Translation is initiated by formation of an initiation complex consisting of the smaller ribosomal subunit, the first amino acid, TRNA, and messenger RNA. The larger ribosomal 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 is called the peptidal, or P-site, and the other is called the acceptor, or A-site. The initiating transfer RNA, carrying its amino acid, 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 amino acid 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. The ribosome now advances a distance of one codon, and the first transfer RNA is released. A transfer RNA carrying the next amino acid now moves into the A-site, and the ribosome shifts down by a distance of one codon. This is followed by the two amino acids on the transfer RNA in the P-site being transferred to the third amino acid. 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 the ribosome reaches a codon that does not code for an amino acid, called a stop codon. The ribosome dissociates into the smaller and larger subunits, and the messenger RNA and protein are released.