THE STRUCTURE OF DNA 



and viscosity measurements. These techniques indicate that DNA is a very 

 asymmetrical structure approximately 20 A wide and many thousands of 

 angstroms long. Estimates of its molecular weight currently center between 5 X 

 10" and 10^ (approximately 3 X 10^ nucleotides). Surprisingly each of these 

 measurements tend to suggest that the DNA is relatively rigid, a puzzhng finding 

 in view of the large number of single bonds (5 per nucleotide) in the phosphate- 

 sugar backbone. Recently these indirect inferences have been confirmed by 

 electron microscopy. Employing high resolution techniques both Williams (1952) 

 and Kahler et al. (1953) have observed, in preparations of DNA, very long thin 

 fibers with a uniform width of approximately 15-20 A. 



II. Evidence for the Existence of Two Chemical Chains 



IN THE Fiber 



This evidence comes mainly from X-ray studies. The material used is the 

 sodium salt of DNA (usually from calf thymus) which has been extracted, puri- 

 fied, and drawn into fibers. These fibers are highly birefringent, show marked 

 ultraviolet and infrared dichroism (Wilkins et al., 1951 ; P>aser and Eraser, 1951), 

 and give good X-ray fiber diagrams. From a preliminary study of these, Wilkins, 

 Franklin and their co-workers at King's College, London (Wilkins et al, 1953; 

 Franklin and Gosling 1953a, b and c) have been able to draw certain general con- 

 clusions about the structure of DNA. Two important facts emerge from their 

 work. They are : 



(1) Two distinct forms of DNA exist. Firstly a crystalline form, Structure A, 

 (Figure 2) which occurs at about 75 per cent relative humidity and contains ap- 

 proximately 30 per cent water. At higher humidities the fibers take up more 

 water, increase in length by about 30 per cent and assume Structure B (Figure 3). 

 This is a less ordered form than Structure A, and appears to be paracrystalline ; 

 that is, the individual molecules are all packed parallel to one another, but are not 

 otherwise regularly arranged in space. In Table 1, we have tabulated some of the 

 characteristic features which distinguish the two forms. The transition from A to 

 B is reversible and therefore the two structures are likely to be related in a simple 

 manner. 



Table 1. 

 (From Franklin and Gosling, 1953a, b and c) 



(2) The crystallographic unit contains two polynucleotide chains. The argument 

 is crystallographic and so will only be given in outline. Structure B has a very 

 strong 3.4 A reflexion on the meridian. As first pointed out by Astbury (1947), 

 this can only mean that the nucleotides in it occur in groups spaced 3.4 A apart in 



195 



