"Codes without commas"

Date:
1957
Reference:
PP/CRI/I/1/12
Catalogue details

Licence: In copyright

Credit: "Codes without commas". Source: Wellcome Collection.

    1/5
    Reprinted from the Proceedings of the National Academy of Sciences, Vol. 43, No. 5, pp. 416-421. May, 1957. CODES WITHOUT COMMAS By F. H. C. Crick, J. S. Griffith, and L. E. Orgel MEDICAL RESEARCH COUNCIL UNIT, CAVENDISH LABORATORY, AND DEPARTMENT OF THEORETICAL CHEMISTRY, CAMBRIDGE, ENGLAND Communicated by G. Gamow, February 11, 1957 This paper deals with a mathematical problem which arose in connection with protein synthesis. We present the solution here because it gives the magic number 20, so that our answer may perhaps be of biological significance. To make this clear, we sketch in the biochemical background first. It is assumed in one of the more popular theories of protein synthesis that amino acids are ordered on a nucleic acid strand (see, for example, Dounce 1 ) and that the order of the amino acids is determined by the order of the nucleotides of the nucleic acid. There are some twenty naturally occurring amino acids commonly found in proteins, but (usually) only four different nucleotides. The problem of how a sequence of four things (nucleotides) can determine a sequence of twenty things (amino acids) is known as the coding problem. This problem is a formal one. In essence, it is not concerned with either the chemical steps or the details of the stereochemistry. It is not even essential to specify whether RNA or DNA is the nucleic acid being considered. Naturally, all these points are of the greatest interest, but they are only indirectly involved in the formal problem of coding. The first definite proposal was made by Gamow. 2 His code, which was suggested by the structure of DNA, was of the overlapping type. The meaning of this is illustrated in Figure 1. Gamow's code was also degenerate—that is, several sets of three letters (picked in a special way) stood for a particular amino acid. However, all the 64 (4 X 4 X 4) possible sets of three letters stood for one amino acid or another, so that any sequence whatever of the four letters stood for a de finite sequence of amino acids. It is easy to see that codes of the overlapping type impose severe restrictions on the allowed amino acid sequences. Unfortunately, no such restrictions have been found, although considerable (unpublished) efforts have been made, by a number of workers, to find them. Part of this work has been reviewed by Gamow, Rich,