I962 M. H. F. WILKIN S 



Fig. 11. Diffraction pattern of transfer RNA showing resolution of diffraction, in the 

 regions of 3.3 A and 4 A, into three layer-lines indicated by the arrows and corres- 

 ponding to the A DNA pattern. (Photograph with W. Fuller and M. Spencer; RNA 



by G. L. Brown.) 



oriented, the three reflections overlapped and gave the impression of two. 

 There was no doubt that the RNA had a regular helical structure almost 

 identical with that of A DNA. The differences between the RNA and 

 DNA patterns could be accounted for in terms of small differences between 

 the two structures. 



An important consequence of the close resemblance of the RNA structure 

 to that of DNA is that the RNA must contain base sequences that are largely 

 or entirely complementary. The number of nucleotides in the molecule is 

 about 80. The simplest structure compatible with the X-ray results consists 

 of a single polynucleotide chain folded back on itself, one half of the chain 

 being joined to the other by base-pairing. This structure is shown in Fig. 12. 

 While we are certain the helical structure is correct, it must be emphasized 

 that we do not know whether the two ends of the chain are at the end of 

 the molecule. The chain might be folded at both ends of the molecule with 

 the ends of the chain somewhere along the helix. It is known that the amino 

 acid attaches to the end of the chain terminated by the base sequence cyto- 

 sine-cytosine-adenine. 



s-53 



