ON THE IMPOSSIBILITY OF ALL OVERLAPPING TRIPLET CODES IN INFORMATION TRANSFER FROM NUCLEIC ACID TO PROTEINS

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Abstract
It is generally accepted that nucleic acids control the synthesis of proteins, and it has been proposed more specifically that the sequence of amino acids in a polypeptide chain is determined by the order of nucleotides in ribo- or deoxyribonucleic acid. The problem of how this determination is effected has come to be known as the "coding" problem. Most codes have been constructed on the basis that each amino acid is determined by a set of three nucleotides. Such triplet codes, however, are sufficient to code the amino acid sequences. It is concluded that all overlapping triplet codes are impossible. This result has one important physical implication. The original formulation of overlapping codes was based on the similarity of the internucleotide distance in DNA to the spacing between amino acid residues in an extended polypeptide chain. It was supposed that each amino acid was spatially related in a one-to-one way with each nucleotide on a nucleic acid template. The present result shows that this cannot be so and that each amino acid is stereochemically related to at least two, if not three, nucleotides, depending on whether coding is partially overlapping or nonoverlapping. The difficulties raised by this can easily be overcome by assuming that the polypeptide sequence is in contact with the nucleic acid template only at the growing point, and detailed schemes can be readily proposed.