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the T. Crosses of the nature outlined could determine amino acid 

 order. However, several difficulties were present. A model of this 

 nature made in three dimensions with actual bond angles, and so on, 

 failed to show any good place to fit the amino acids. The model used 

 each purine or pyridine in two crosses so that there would be an 

 appreciable influence of one amino acid on the next; this has not been 

 found in natural proteins. Finally, other biological data favor control 

 of the synthesis of RNA by DNA. In turn, RNA is involved in protein 

 synthesis. In spite of its obvious limitations, Gamow's model indicated 

 information could be coded in DNA. 



Figure 14. Gamow's model of DNA. After G. Gamow, 

 "Information Transfer in the Living Cell," Scientific American 

 193: 70 (1955). 



X-ray diffraction patterns have been studied for RNA as well as 

 DNA. The structure of RNA is more complicated than that of DNA. 

 The ratio of purine to pyrimidine is not one in RNA. The RNA chains 

 may be branched. Its X-ray diffraction pattern suggests a single-chain 

 helical symmetry. The detailed interpretation of this pattern still has 

 not been accomplished. 



In spite of this failure to determine the details of the structure of the 

 RNA molecule, X-ray diffraction and electron scattering have been used 

 to locate the nucleic acid within the RNA-type virus particles. This 

 may be done for any of a considerable number of viruses which have 

 been crystallized. The very existence of crystals implies detailed 

 ordering at the atomic level, and accordingly a definite X-ray diffraction 

 pattern. Figure 15 shows a model of tobacco mosaic virus (TMV) 

 constructed to fit X-ray diffraction data. More recent models of TMV 

 reveal that the RNA is held in a large vacuous helix similar to the DNA 



