AUG is a start codon that codes for methionine whereas UGA, UAA and UAG are stop codons. The Initiator codon is the first codon of a messenger RNA or mRNA transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes and a modified methionine, i.e., fMet in prokaryotes. Termination codons or stop codon is a nucleotide triplet within messenger RNA that signals translational termination.