J. D. WATSON AND F. H. C. CRICK 



the fiber direction. On going from Structure B to Structure A the fiber shortens 

 by about 30 per cent. Thus in Structure A the groups must be about 2.5 per cent 

 A apart axially. The measured density of Structure A, (Frankhn and Goshng, 



Figure 2. X-ray fiber diagram of structure A of desoxyribonucleic acid. (H. M. 

 F. Wilkins and H. R. Wilson, unpub.) 



1953c) together with the cell dimensions, shows that there must be two nucleo- 

 tides in each such group. Thus it is very probable that the crystallographic unit 

 consists of two distinct polynucleotide chains. Final proof of this can only come 

 from a complete solution of the structure. 



Structure A has a pseudo-hexagonal lattice, in which the lattice points are 22 A 

 apart. This distance roughly corresponds with the diameter of fibers seen in the 

 electron microscope, bearing in mind that the latter are quite dry. Thus it is 

 probable that the crystallographic unit and the fiber are the one and the same. 



III. Description of the Proposed Structure 

 Two conclusions might profitably be drawn from the above data. Firstly, the 



196 



