3 Amino acids in a polypeptide chain were found to be coded by groups of three nucleotides in a mrNa Simple calculation indicated that three or more bases are probably needed to specify one amino acid Genetic studies of insertion deletion and substitution mutants showed codons for amino acids are triplet of nucleotides; codons do not overlap and there is no punctuation between codons for successive amino acid residues The amino acid sequence of a polypeptide is defined by a linear sequence of contiguous codons: the first codon establishs a reading frame3. Amino acids in a polypeptide chain were found to be coded by groups of three nucleotides in a mRNA • Simple calculation indicated that three or more bases are probably needed to specify one amino acid. • Genetic studies of insertion, deletion, and substitution mutants showed codons for amino acids are triplet of nucleotides; codons do not overlap and there is no punctuation between codons for successive amino acid residues. • The amino acid sequence of a polypeptide is defined by a linear sequence of contiguous codons: the first codon establishs a reading frame