We can divide translation, the process of building a polypeptide, into three stages: initiation, elongation, and termination. Translation is initiated when the small ribosomal subunit binds to the leader at the 5 prime end of the mRNA molecule. The anticodon of the initiator tRNA binds to the start codon, AUG. The initiator tRNA always bears the amino acid methionine. Proteins called initiation factors help bring the mRNA, the initiator tRNA, and the small ribosomal subunit together with the large ribosomal subunit to form an initiation complex. The initiator tRNA sits in the P site on the ribosome.