Reprints 56-65

Date:
1964-1968
Reference:
PP/CRI/M/1/3
Catalogue details

Licence: In copyright

Credit: Reprints 56-65. Source: Wellcome Collection.

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    ». * s 4 iMunf —J / 43 CHAPTER III GENERAL NATURE OF THE GENETIC CODE* rF.jJL'C. C rick, University of Cambridge, Cambridge, England I .have taken as my title The General Nature of the Genetic Code ,Jbeeauae, we do ,not yet .know .the genetic code in .detail, although we certainly hope to do.so before very long. ¡Relieve ; that genes are made of nucleic §cjd. ft p f eed not concern us ^t t this^tage whether it is DNA or RNA, double.-str^nded or single-stranded. ^e also believe that one of the main functions of genes is to determine the .amino acid sequence of proteins. There may be other functions but this function is certainly an important one. In addition, proteins have to take up their correct secondary and tertiary structure. I shall assume here without further discussion that the .folding of a protein is mainly dictated by its primary structure. The genetic code, then, covers the problem of how the sequence of bases in a particular piece of nucleic acid determines the sequence of amino acids in a particular protein. There appear to be four common bases in nucleic acid: guanine, adenine, thymine (or uracil) and cytosine. It is true there are a few rarer ones, most of which can be described as having additional methyl or hydroxymethyl groups. We cannot be sure that they do not have ^Pine important function in the genetic code, but this is not very likely. Jt is an interesting speculation as to whether we could have nucleic acid with six bases; that is, three base-pairs instead of two base-pairs. Dr. Alex Rich, a little time ago, did make a suggestion along these lines (Rich, 1963), bui we can say fairly categorically that at the moment we have no indication of a third base-pair in any biological material. Tlie polypeptide chains of proteins are made from a standard set of ^v«nty amino acids. Again you could argue about the number twenty. It does not include hydroxyproline, phosphoserine, and certain other amino acids which are certainly found in true proteins. Nevertheless, for reasons which I have explained elsewhere r ( Crick, 1963), I think the standard set of twenty is probably the correct one. I shall assume, therefore, that we have four standard bases in nucleic acids and twenty standard amino acids in proteins. Moreover the bases and *An address presented before The Robert A. Welch Foundation Conferences on Chemical Research. VIII. Selected Topics in Modern Biochemistry, which was held in Houston, Texas, November 16-18, 1964.
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