78 



JERRY DONOHUE 



together if the two chains are complementary. Fig. 19 shows a corresponding 

 possibihty in the case of the pyrimidines, thymine and cytosine. 



Systematic attack of this problem has shown (Donohue, 1956) that there are 

 24 possible pairings, and that of these, there are 5 pairs of pairs which can give 

 rise to complementary two chain structures. One of these is the Watson-Crick 

 DNA structure (Fig. 16), and Figs. 17, 18 and 19 show three of the others. A 

 remaining one allows all four bases, like those of Figs. 16 and 17, but the com- 

 plementariness is opposite to them, that is, adenine pairs with cytosine and 

 guanine with thymine. 



None of these, other than the Watson-Crick structure which is very probably 

 the correct one for DNA, have as yet been shown to have any significance to 

 the nucleic acid problem. In the future, however, samples of another kind of 



Fig. 17. An alternate method of pairing the bases in nucleic acid which predicts 

 the same analytical composition as the Watson-Circle structure, Fig. 16, but, if the 

 two polynucleotide chains run in the same direction, gives a structure which is not in 

 agreement with the X-ray data for DNA. 



