Translation is the process by which the information contained in messenger RNA is used to direct the synthesis of a polypeptide. This information is carried in the sequence of bases on the messenger RNA molecule. The polypeptide can be assembled once RNA binds to a ribosome. Ribosomes consist of large and small subunits. The large subunit has 2 binding sites for transfer RNA, the P and A sites. Transfer RNA carries an amino acid, which will be incorporated into the polypeptide. Its anticodon is a triplet complementary to the codon on messenger RNA that specifies that particular amino acid. A ribosome assembles on the start codon, AUG. Transfer RNA with the anticodon UAC and carrying the amino acid methionine, binds to the codon. The transfer RNA is in the P site of the ribosome. The A site is available for a second transfer RNA with an anticodon complementary to the second messenger RNA triplet. The amino acid carried by the second transfer RNA binds to the methionine. The first transfer RNA leaves the P site and the second transfer RNA moves there, still bound to messenger RNA. This brings the third messenger RNA codon to the now-empty A site and the appropriate transfer RNA can bind to it. The third amino acid is added to the chain and translocation occurs once again. This process of polypeptide chain elongation continues until a stop codon is reached. A release factor binds to the A site. It carries no amino acid, but facilitates release of the polypeptide and the messenger RNA from the ribosome.