1962 M. H. F.WILKIN S 



Contour interval 2C|A 

 zero contour dashed 



Fig. 7. Fourier synthesis map (by S. Arnott) showing the distribution of electron 

 density in the plane of a base-pair in the B configuration of DNA. The distribution 

 corresponds to an average base-pair. The shape of the base-pair appears in the map, but 

 individual atoms in a base-pair are not resolved. (The Fourier synthesis is being revised 

 and the map is subject to improvement.) 



been of this nature. To be reasonably certain that the DNA structure was 

 correct, X-ray data, as extensive as possible, had to be collected. 



The B configuration 



Fig. 5 shows a diffraction pattern of a fibre of DNA at high humidity when 

 the molecules are separated by water and, to a large extent, behave independ- 

 ently of each other. We have not made intensive study of DNA under these 

 conditions. The patterns could be improved, but they are reasonably well- 

 defined, and the sharpness of many of their features shows that the molecules 

 have a regular structure. The configuration is known as B (see also Fig. 3); 

 it is observed in vivo, and there is evidence that it exists when DNA is 111 

 solution in water. There are 10 nucleotide pairs per helix turn. There is no 

 obvious structural reason why this number should be integral; if it is exactly 

 so, the significance of this is not yet apparent. 



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