THE STRUCTURE OF DNA 



basic structure is helical (Wilkins et al., 1953; Franklin and Gosling, 1958a). If 

 we postulate that a helix is present, we immediately are able to deduce from the 

 X-ray pattern of Structure B (Figure 3), that its pitch is 34 A and its diameter ap- 

 proximately 20A. Moreover, the pattern suggests a high concentration of atoms 

 on the circumference of the helix, in accord with our model which places the 

 phosphate sugar backbone on the outside. The photograph also indicates that 

 the two polynucleotide chains are not spaced equally along the fiber axis, but are 

 probably displaced from each other by about three-eighths of the fiber axis 

 period, an inference again in qualitative agreement with our model. 



The interpretation of the X-ray pattern of Structure A (the crystalline form) is 

 less obvious. This form does not give a meridional reflexion at 3.4 A, but instead 

 (Figure 2) gives a series of reflexions around 25° off the meridian at spacings be- 

 tween 3 A and 4 A. This suggests to us that in this form the bases are no longer 

 perpendicular to the fiber axis, but are tilted about 25° from the perpendicular 

 position in a way that allows the fiber to contract 30 per cent and reduces the 

 longitudinal translation of each nucleotide to about 2.5 A. It should be noted 

 that the X-ray pattern of Structure A is much more detailed than that of Struc- 

 ture B and so if correctly interpreted, can yield more precise information about 

 DNA. Any proposed model for DNA must be capable of forming either Struc- 

 ture A or Structure B and so it remains imperative for our very tentative interpre- 

 tation of Structure A to be confirmed. 



(2) The anomolous titration curves of undegraded DNA with acids and bases 

 strongly suggests that hydrogen bond formation is a characteristic aspect of- DNA 

 structure. When a solution of DNA is initially treated with acids or bases, no 

 groups are titratable at first between pH 5 and pH 11.0, but outside these limits 

 a rapid ionization occurs (Gulland and Jordan, 1947; Jordan, 1951). On back 

 titration, however, either with acid from pH 12 or with alkali from pH 2^, a dif- 

 ferent titration curve is obtained indicating that the titratable groups are more 

 accessible to acids and bases than is the untreated solution. Accompanying the 

 initial release of groups at pH 1 1.5 and in the range pH 3.5 to pH 4.5 is a marked 

 fall in the viscosity and the disappearance of strong flow birefringence. While 

 this decrease was originally thought to be caused by a reversible depolymerization 

 (Vilbrandt and Tennent, 1943), it has been shown by Gulland, Jordan and Taylor 

 (1947) that this is unlikely as no increase was observed in the amount of sec- 

 ondary phosphoryl groups. Instead these authors suggested that some of the 

 groups of the bases formed hydrogen bonds between different bases. They 

 were unable to decide whether the hydrogen bonds linked bases in the same or in 

 adjacent structural units. The fact that most of the ionizable groups are orig- 

 inally inaccessible to acids and bases is more easily explained if the hydrogen 

 bonds are between bases within the same structural unit. This point would defi- 

 nitely be established if it were shown that the shape of the initial titration curve 

 was the same at very low DNA concentrations, when the interaction between 

 neighboring structural units is small. 



(3) The analytical data on the relative proportion of the various bases show 

 that the amount of adenine is close to that of thymine, and the amount of guanine 



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