There is a start codon which is present at the first position in the mRNA sequence. This start codon is the initiation point for the polypeptide synthesis. The start codon which is present may be AUG, GUG or UUG. These start codons code for methionine in eukaryotes and formyl methionine in prokaryotes. This is the first amino acid which gets introduced in the polypeptide chain during the synthesis of the protein by the ribosome.