"The origin of the genetic code"

Date:
1967
Reference:
PP/CRI/H/4/9
Catalogue details

Licence: In copyright

Credit: "The origin of the genetic code". Source: Wellcome Collection.

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    368 F. H. C. CRICK Table 1 The genetic code 1st I 2nd PHE SER TYR CYS U PHE SER TYR CYS C LEU SER Ochre V A LEU SER Amber TRP G LEU PRO HIS ARG U LEU PRO HIS ARG C LEU PRO GLN ARG A LEU PRO GLN ARG G ILE THR ASN SER U ILE THR ASN SER C ILE THR LYS ARG A MET THR LYS ARG G VAL ALA ASP GLY U VAL ALA ASP GLY C VAL ALA GLU GLY A VAL ALA GLU GLY G This Table shows the best allocations of the 64 codons at the time of the Symposium. Some of these allocations are less certain than others. The two codons marked ochre and amber are believed to signal the termination of the polypeptide chain. The codons suspected of being con cerned with chain initiation are not indicated here. Again the function of the three presumed nonsense triplets is not known for certain. It is presumed that UAA (ochre) and UAG (amber) are signals for chain termination and probably UGA as well, at least in bacteria. In E. coli there appears to be a special mechanism for initiating the polypeptide chain, involving formylmethionine and the codons AUG and GUG. The mechanism in higher organisms (if indeed a special one exists) is unknown. Finally, it is uncertain whether there are ambiguous codons; that is, codons which represent more than one amino acid. Of course, it is known that mutations can produce errors in the translation mechanism and so make certain codons ambiguous, but it is not known whether ambiguity occurs normally. Again in what follows I shall assume that this is not usually the .case for present-day organisms. The basic reason why one can ignore these complications and uncertainties for the moment is that the broad features of the genetic code are not likely to be greatly affected by them. What, then, are the properties of the code which require explana tion ? There are some features which are of such a general type that they do not depend at all upon the details of the code. They are: (1) there are 4 distinct bases in the m R ISTA, (2) each codon is a triplet of bases, (3) only 20 of the numerous possible amino acids are used. In examining Table 1, however, one is apt to take all these characteristics for granted. What, then, is special about the actual details of the genetic code ?
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