HYDROGEN BONDING 



77 



lytical results that in DNA the adenine content equals the thymine content, 

 and the guanine content equals the cytosine content, even though the ratio 

 of thymine to cytosine may vary widely from one source of DNA to another. 

 The structure shown in Fig. 17 will also lead to this composition. This structure, 

 however, has a two-fold a.xis coincident with the helical axis, instead of perpen- 

 dicular to it as in the Watson-Crick structure, and is therefore not in agreement 

 with the X-ray data for DNA. 



If we restrict ourselves to two-chain structures involving only pairs of bases, 

 there are a number of other possibilities. Fig. 18 shows the arrangement for 

 chains which contain only the two purines, adenine and guanine. This repre- 

 sents the basis of a molecule containing adenine alone, guanine alone, or both 



Fig. 16. The pairing of the four bases, adenine (A), thymine (T), guanine (G), 

 and cytosine (C), in the Watson-Circle deoxyribonucleic acid structure. Nitrogen 

 atoms are shown as filled circles, ox\'gen atoms as open circles, hydrogen bonds as 

 dashed lines. The bonds from the bases to the sugar residues are shown as heavy lines. 

 This structure agrees well with the X-ray data. 



