MOLECULAR CONFIGURATION OF NUCLEIC ACIDS 



Fig. 8. Molecular model of DNA in the A configuration. The base-pairs may be seen 

 inclined 20° to the horizontal. 



When DNA crystallizes, the process of crystallization imposes restraints 

 on the molecule and can give it extra regularity. Also, the periodic arrange- 

 ment of the molecules in the microcrystals in the fibre causes the diffraction 

 pattern to be split into sharp reflections corresponding to the various crystal 

 planes (Fig. 6). Careful measurement of the positions of the reflections and 

 deduction of the crystal lattice enables the directions of the reflections to be 

 identified in three dimensions. Diffraction patterns from most fibrous sub- 

 stances resemble Fig. 5 in that the diffraction data arc 2-dimcnsional. In 

 contrast, the crystalline fibres of DNA give fairly complete 3 -dimensional 

 data. These data give information about the appearance of the molecule 

 when viewed from all angles, and are comparable with those from single 



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