MOLECULAR CONFIGURATION OF NUCLEIC ACIDS 



Fig. 13. Diffraction pattern of unoricntcd transfer RNA, showing diffraction rings 



with spots corresponding to reflections from single crystals of RNA. The arrows point 



to reflections from planes ~ 6 A apart. 



number of bases in a DNA molecule is too large for determination of base 

 sequence by X-ray diffraction to be feasible. However, in transfer RNA the 

 number of bases is not too large. The possibility of complete structure anal- 

 ysis of transfer RNA by means of X-rays is indicated by two observations. 

 First, we have observed (Fig. 13), in X-ray patterns of transfer RNA, sep- 

 arate spots each corresponding to a single crystal of RNA. We estimated 

 their size to be about 10^ and have confirmed this estimate by observing, in 

 the polarizing microscope, bircfringent regions that probably arc the crys- 

 tals. It should not be too difficult to grow crystals several times larger, which 

 is large enough for single-crystal X-ray analysis. 



The second encouraging observation is that the X-ray data from DNA 

 have restricted resolution almost entirely on account of disorientation of the 

 microcrystals in DNA fibres. The DNA intensity data indicate that the 

 temperature factor (B = 4 A) is the same for DNA as for simple compounds. 

 It thus appears that DNA crystals have fairly perfect crystallinity and that, if 



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