THE STRUCTURE OF DMA 



structure of DNA is regular enough to form a three dimensional crystal. This 

 is in spite of the fact that its component chains may have an irregular sequence of 

 purine and pyrimidine nucleotides. Secondly, as the structure contains two 



Figure 3. X-ray fiber diagram of Structure B of desoxyribonucleic acid. 

 Franklin and R. Gosling, 1953a.) 



(R. E. 



chains, these chains must be regularly arranged in relation to each other. 



To account for these findings, we have proposed (Watson and Crick, 1953a) a 

 structure in which the two chains are coiled round a common axis and joined to- 

 gether by hydrogen bonds between the nucleotide bases (see Figure 4). Both 

 chains follow right handed helices, but the sequences of the atoms in the phos- 

 phate-sugar backbones run in opposite directions and so are related by a dyad 

 perpendicular to the helix axis. The phosphates and sugar groups are on the out- 

 side of the helix whilst the bases are on the inside. The distance of a phosphorus 

 atom from the fiber axis is 10 A. We have built our model to correspond to Struc- 

 ture B, which the X-ray data show to have a repeat distance of 34 A in the fiber 

 direction and a very strong reflexion of spacing 3.4 A on the meridian of the X-ray 



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