M. H. F. WILKINS 



Figure 3 shows a section at right angles to the helix axis and in the plane 

 of a base-pair. The position of the atoms in the model are marked. Since 

 the sequence of bases along a polynucleotide chain is not periodic, the 



Fig. 4. Fourier difference synthesis through same section as Fig. 3. The 

 space between DNA molecules has been treated approximately as uniformly filled 

 with water. There is almost no indication that the separation of the glycosidic 

 links needs to be altered. If the Watson-Crick pairs were incorrect and the 

 Hoogsteen scheme correct, the region near AA should be positive and that at BB 

 negative. In fact the signs are the reverse. 



X-ray diffraction method shows an average base. It may be seen that 

 regions of high electron density correspond to the positions of atoms in the 

 base and deoxyribose parts of the molecule. The distance between the 

 glycosidic links appears somewhat larger on the Fourier section than on 

 the model. However, the synthesis was performed without taking account 

 of the presence of water in the structure. When the DNA molecule is 



