I962 M. H. F.WILKIN S 



Fig. 2. Watson-Crick base-pairs (revised by S. Arnott). (Top): Guanine hydrogen- 

 bonded to cytosine. (Bottom): Adenine hydrogen-bonded to thymine. The distances be- 

 tween the ends of the Q-N3 and Q-N, bonds are 10.7 A in both pairs, and all these 

 bonds make an angle of 52 with the Q-Cj line. 



joined by the base-pairs, the distance between the two chains is the same for 

 both base-pairs and, because the angle between the bonds and the Q-Cj 

 line is the same for all bases, the geometry of the deoxyribose and phosphate 

 parts of the molecule can be exactly regular. 



Watson and Crick built a two-chain molecular model of this kind, the 

 chains being helical and the main dimensions being as indicated by the X-ray 

 data. In the model, one polynucleotide chain is twisted round the other and 



s-37 



