MO LECULAB CON FI G U RATION OF NUCLEIC ACIDS 



unusually small for a nucleic acid, there were indications that it might have 

 a regular structure, its biochemical role was important, and m many ways 

 its functioning was understood. 



We found it very difficult to orient transfer RNA in fibres. I Iowever, by 

 carefully stretching RNA gels in a dry atmosphere under a dissecting mi- 

 croscope, 1 found that fibres with birefringence as high as that of DNA 

 could be made. But these fibres gave patterns no better than those obtained 

 with other types of RNA, and the molecules disoriented when the water 

 content of the fibres was raised. Watson, Fuller, Michael Spencer, and my- 

 self worked for many months trying to make better specimens for X-ray 

 study. We made little progress until Spencer found a specimen that gave 

 some faint but sharp diffraction rings in addition to the usual diffuse RNA 

 pattern. This specimen consisted of RNA gel that had been sealed for X-ray 

 study in a small cell, and he found that it had dried slowly owing to a leak. 

 The diffraction rings were so sharp that wc were almost certain that they 

 were spurious diffraction due to crystalline impurity - this being common 

 in X-rav studies of biochemical preparations. A specimen of RNA had given 

 very similar rings due to DNA impurity. Wc were therefore not very 

 hopeful about the rings. However, after several weeks Spencer eliminated 

 all other possibilities: it seemed clear that the rings were due to RNA itself. 

 By controlled slow drying, he produced stronger rings; and, with the refined 

 devices wc had developed for stretching RNA and with gels slowly con- 

 centrated by Brown, Fuller oriented the RNA without destroying its crys- 

 talhnity. These fibres gave clearly defined diffraction patterns, and the ori- 

 entation did not disappear when the fibres were hydratcd. It appeared that 

 the methods I had been using earlier, of stretching the fibres as much as 

 possible, destroyed the crystallinity. If instead, the material was first allowed 

 to crystallize slowly, stretching oriented the microcrystals and the RNA 

 molecules in them. Single molecules were too small to be oriented well 

 unless aggregated by crystallization. It was rather unexpected that, of all the 

 different types of RNA we had tried, transfer RNA which had the lowest 

 molecular weight, oriented best. 



The diffraction patterns of transfer RNA were clearly defined and well- 

 oriented (Spencer, Fuller, Wilkins, and Brown 24 ). These improvements re- 

 vealed a striking resemblance between the patterns of RNA and A DNA 

 (Fig. 10). The difficulty of the two reflections at 3.3 A and 4 A was resolved 

 ( Fig. 1 1 ) : in the RNA pattern the positions of reflections on three layer-lines 

 differed from those in DNA; as a result, when the patterns were poorly 



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