Initiation codon (i) This is the triplet codon at which polypeptide synthesis begins. (ii) It is always AUG and codes for a methionine. (iii) As a result, all polypeptides are synthsized with an amino-terminal methionine. Coding region (i) This is the sequence of mRNA that contains the consecutive triplet codons that direct polypeptide synthesis. (ii) This region spans from the start codon to the stop codon. (iii) The coding region is often referred to as the open reading frame or ORF. Stop codon (i) This is the triplet codon that signals the termination of translation. (ii) There are three possible stop codon sequences: UAA, UAG, UGA. (iii) Stop codons have no corresponding tRNA or amino acid.
