VIRUSES AS MOLECULES 



11 



chemical properties of the material and which are all of the same size and 

 shape, a requirement necessitated by the stipulation that all molecules of a 

 given substance be made up of the same number of atoms arranged in the same \:&y. 



It has already been seen from the electron microscope pictures that to- 

 bacco mosaic virus is particulate. The early work of Stanley, some of which was 

 reviewed, showed most elegantly that these particles cannot be broken down into 

 smaller units without destroying their most fundamental property,- infectivity. 

 It is necessary only to demonstrate that these particles are all of the aame 

 size and shape in order to demonstrate that they can be molecules. If the 

 electron micrograph of the virus is reconsidered, it can be seen that all of the 

 particles are of the same shape, but alas, they are not quite all of the same 

 size. Some are just a little longer than others. Hence we have failed on the 

 very last step to demonstrate that tobacco mosaic virus particles are molecules 

 in the strictest sense of the term. Some of us believe that some day it may be 

 possible to obtain tobacco mosaic virus preparations which meet even this final 

 criterion of homogeneity. As a matter of fact, preparations which are very 

 nearly homogeneous have recently been obtained. However, for the present it is 

 necessary to consider tomato bushy stunt virus in order to demonstrate that 

 viruses can be protein molecules. 



Bushy stunt virus crystallizes in the form of beautiful dodecahedra, as 

 illustrated in Figure ?• 



FIGURE 7 - CRYSTALS OF TOMATO BUSHY STUNT VIRUS, 

 (W.M.Stanley, J. Biol. Chem. 135, 437 (194-0) ). 



