Polypeptide chain synthesis is signaled by two initiation codons AUG or GUG.
AUG codes for Methionine amino acid in Eukaryotes and In Prokaryotes, AUG codes for N–formyl methionine.
Sometimes, GUG also functions as a start codon. It codes for Valine amino acid normally but when it is present at starting position it codes for Methionine amino acid.
Out of 64 codons 3–codons are stopping or nonsense or termination codon.
Nonsense codons do not specify any amino acid