26 



M. H. F. WILKINS 



synthesis method as described below. Stereochemical difficulty has been 

 found in placing guanine-cytosine pairs in the model [19] but these difficul- 

 ties may not be insurmountable. 



WATSON- CRICK S-S = 



HOOGSTEEN S-S=9'2 A MAXIMUM 



Fig. 10. Comparison of the Watson-Crick and Hoogsteen base-pairing 

 arrangements. The hexagonal purine ring is at A' and 1' in the two schemes. 



Use of the Fourier synthesis method to assess the probability that 

 base-pairs in DNA are not of the Watson Crick type 



We have built complete molecular models of DNA with Watson-Crick 

 base-pairs [3, 4]. The Fourier synthesis method enables us to assess the 

 probability that the base pairs are of a different type without constructing 



