270 



( HAP I BR 20 



i k,i ki 20—6. ///<• opposite direction of the sugar-phosphate 

 linkages in the two strands of a DNA double helix. 



5'CH 



H 2 C5' 



that A in one chain can pair only with T in 

 the other chain — similarly C with G — to 

 form a double helix of constant diameter 

 whose strands are held together by the maxi- 

 mum number of H bonds. Since A and T 

 always go together (as do C and G), the 

 equivalences A = T and C = G, derived 

 from chemical analysis of DNA, become 

 meaningful as the direct consequence of the 



secondary structure of DNA. In fact, these 

 chemical equivalences provide the first in- 

 dependent test of the Watson-Crick model 

 constructed initially on the basis of the X-ray 

 diffraction diagrams among other considera- 

 tions. 



To form the maximum number of H bonds 

 between a purine and a pyrimidine, it is 

 necessary to represent one of the two as 



