Experimental Evidence for the Degeneracy of the Nucleotide Triplet Code

EXTENSIVE investigations have been reported recently of the incorporation of labelled amino-acids in cell-free protein-synthesizing systems to which synthetic polynucleotides of known compositions were added. The results have led to the assignment of coding triplets to each of the 20 naturally occurring amino-acids1,2. The peculiar feature of these codes is the presence of at least one uracil residue in each of the 20 triplets.