ON THE GENETIC CODE 



Comma-less triplet codes 



All such codes arc unlikely, not only because of the genetic evidence but 



also because of the detailed results from the cell-free system. 



Two-letter or three-letter codes 



For example a code in which A is equivalent to O, and G to U. As al- 

 ready stated, the results from the cell-free system rule out all such codes. 



The combination triplet code 



In this code all permutations of a given combination code the same amino 

 acid. The experimental results can only be made to fit such a code by very 

 special pleading. 



Complementary codes 



There are several classes of these. Consider a certain triplet in relation to the 

 triplet which is complementary to it on the other chain of the double helix. 

 The second triplet may be considered either as being read in the same direc- 

 tion as the first, or in the opposite direction. Thus if the first triplet is UCC, 

 we consider it in relation to either AGG or (reading in the opposite direction) 

 GGA. 



It has been suggested that if a triplet stands for an amino acid its comple- 

 ment necessarily stands for the same amino acids, or, alternatively in another 

 class of codes, that its complement will stand for no amino acid, i.e. be 

 nonsense. 



It has recently been shown by Ochoa's group that poly A stimulates the 

 incorporation of lysine 10 . Thus presumably AAA codes lysine. However 

 since UUU codes phenylalanine these facts rule out all the above codes. It is 

 also found that poly (U,G) incorporates quite different amino acids from 

 poly (A,C). Similarly poly (U,C) differs from poly (A,G) 9 - 10 . Thus there 

 is little chance that any of this class of theories will prove correct. Moreover 

 they are all, in my opinion, unlikely for general theoretical reasons. 



A start has already been made, using the same polynucleotides in cell-free 

 systems from different species, to see if the code is the same in all organisms. 

 Eventually it should be relatively easy to discover in this way if the code is 

 universal, and, if not, how it differs from organism to organism. The prelim- 

 inary results presented so far disclose no clear difference between E. coli and 

 mammals, which is encouraging 10 ' 13 . 



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