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X-ray Analyses of Proteins and Nucleic Acids / 1 5 : 7 



helix but with quite different physical dimensions. Instead of being 

 supported by hydrogen bridges, the RNA helix in TMV is held in place 

 and stabilized by the surrounding protein molecules which actually 

 intermesh with the RNA (this is not illustrated in Figure 15). 



RNA Helical Core 



3,000 X 



Protein Coat 



2,500 Globular Molecules 



Wound in Helices 



170 A 



Figure 15. The structure of tobacco mosaic virus. The figure 

 pictorially represents results of X-ray diffraction data. 



Another plant virus whose gross structure has been determined is 

 bushy stunt tomato virus (BSV) . The particles of BSV are spherical, 

 but, just as TMV, consist of a nucleic acid core plus a surrounding 

 envelope of protein. These structures for TMV and BSV are in accord 

 with the view of virus activity presented in the previous chapter. 



7. Summary 



The X-ray study of the structure of living matter is a fascinating and 

 growing field. It has made possible discoveries of the steric form of 

 complex high polymers such as proteins and nucleic acids, both of 

 which are responsible for many of the properties of all living systems. 

 These studies, based on X-ray diffraction, have revealed both a com- 

 plexity that was previously beyond imagination, and a simplicity and an 

 ordering of atoms on a much larger scale than had been previously sus- 

 pected. Analyses at the molecular level have contributed many of the 

 major steps taken in recent years to the understanding of living matter. 

 The advances discussed in this chapter are not the result of X-ray 

 diffraction studies alone; rather, a great many divergent approaches 



