Genetic code is a triplet code:

In During translation, the series of an mRNA molecule is read from its 5' end through ribosomes that then synthesize an appropriate polypeptide. Together in prokaryotes and in eukaryotes the DNA sequence of a one gene is colinear with the amino acid sequence of the polypeptide it encodes. Alternatively, the nucleotide sequence of the coding DNA strand, 5' to 3', specifies in specifically the similar order the amino acid sequence of the encoded polypeptide N-terminal  to C-terminal. The relationship among the nucleotide sequence of the mRNA and the  amino  acid  sequence  of  the  polypeptide  is  known as  the  genetic  code.  The series  of the  mRNA  is read  in sets  of three  nucleotides  called  codons, by  each  codon  specifying  a  particular  amino  acid that was shown in the figure.  Moreover, three codons, UGA, UAG and UAA, do not encode an amino acid. Whenever one of these codons is encountered through a ribosome, it leads to termination of protein synthesis.  Thus these three codons are known termination codons or stop codons. The codon AUG codes for methionine. While methionine is found at internal  positions  in polypeptide  chains all eukaryotic  polypeptides  also begin with  methionine and all prokaryotic  polypeptides  begin  with  a modified  methionine  (N-formyl  methionine;). Thus, the first AUG codon that is read through the ribosome in an mRNA is known as the initiation codon or begins codon