20 



M. H. F. WILKINS 



The Watson-Crick base-pairing schemes and possible alternatives 



The Watson-Crick scheme [8, 9, 10] is shown in Fig. 5, It should be 

 noted that the glycosidic hnks in both base-pairs are arranged symmet- 

 rically about a line (dyad axis) in the plane of the diagram and passing 



Fig. 6. The Donohue base-pairs [10]. 



through the helix axis. As a result the molecule has the same appearance 

 (apart from the sequence of bases) when turned upside down, i.e. when 

 rotated 180° about the dyad axis. In both pairs the separation of the 



