VIRUSES 



joined in the extensive use of the newly developed electron microscope. 

 This instrument covers the entire range of sizes occupied by viruses 

 and has proved, and will doubdess continue to prove, of the greatest 

 value in the estimation of the sizes of viruses. In those cases in which 

 more than one method has been employed, good agreement has 

 usually been secured. The occasional discrepancies have been found 

 to be due to errors or to a failure to appreciate the limitations of a given 

 method, and generally have been resolved. At present the sizes of 

 several viruses are well established and the values are accepted by both 

 chemists and pathologists. 



Tobacco mosaic, the first virus to be discovered, was also the 

 first virus to be prepared in essentially pure form and subjected to 

 extensive chemical study. No difference was noted in virus samples 

 prepared from a wide variety of hosts or from the same kind of host at 

 different times of the year. The virus particles were found to consist 

 of about 6% nucleic acid of the ribose type and about 94% protein. 

 The exact nature of the linkage between protein and nucleic acid is 

 unknown, but it appears to be considerably stronger than that which 

 exists in the sperm nucleoproteins. The protein component contains 

 definite and reproducible amounts of over thirteen amino acids. It is 

 of interest that, in contrast to the sperm nucleoproteins, there does not 

 appear to be an excess of basic amino acids. The single virus particles 

 are about 280 m^u in length and 15 m^t in diameter. Tobacco mosaic 

 virus activity has never been demonstrated to be associated with smaller 

 particles. However, there is good evidence that a single virus particle 

 is built up from similar subunits fitted together in a hexagonal lattice 

 to yield the final structure which possesses virus activity. The nucleic 

 acid of this final structure appears to exist in the form of eight threadlike 

 units laid down along the length of the particle. Because of the repeat 

 pattern within a single virus particle, it can be regarded as a sub- 

 microscopic crystal. In addition, these single virus particles can 

 aggregate with a two-dimensional regularity to form large needlelike 

 crystals that are readily visible with a low-power hand lens. Of 

 especial interest and significance is the fact that the virus particles 

 appear to be utterly devoid of water and of any enzymic activity 

 other than virus activity. The complete lack of water and the crystal- 

 like inner structure of the individual virus particles would appear to 

 preclude the existence of metabolism of the type usually associated with 



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