Because RNA is composed  of four parts of nucleotides,  there are 43 =  64 possible codons,  which  is  64  possible  triplets  of  nucleotides   with  several  sequences. Furthermore, only 20 amino acids are commonly found in proteins so which, in most cases, a one amino acid is coded for through various dissimilar codons described in the figure. The genetic code is thus said to be degenerate.  In reality, only methionine and tryptophan are shown by a single codon. As a conclusion of the genetic code's degeneracy, a mutation which changes only a single nucleotide in

                                       Figure:  The genetic code.

DNA point mutation, and thus changes only a single nucleotide in the corresponding mRNA, frequently has no effect on the amino acid sequence of the encoded polypeptide.