VIRUSES 



has been possible foi' the pathologist to recognize, separate, and grow 

 individual virus strains. Some of these have been found of girnt use- 

 as vaccines for the prevention of certain virus diseases. 



In view of the large size and complexity of structure of \inises, ii 

 may appear that chemists were somewhat optimistic in expecting to 

 be able to detect differences in the chemical structure of different strains 

 of a virus. Two similar large mountains may appear identical when 

 viewed from a distance, but close inspection will reveal differences. 

 So, too, with the viruses. The over-all structures of different strains 

 of tobacco mosaic virus were found to be very similar; yet in several 

 instances it has been possible to demonstrate definite differences in 

 chemical structure. These consist of differences in the amount of one 

 or more amino acids, in the presence of an entirely new amino acid, 

 or in the complete elimination from the virus structure of a given amino 

 acid. These changes represent deep seated and fundamental altera- 

 tions in the virus structure, and it seems unlikely that they could have 

 resulted from alterations of fully formed virus particles. It appears 

 more likely that these changes occurred as a result of a dixersion of the 

 synthetic process by means of which a virus reproduces. Since new 

 strains tend to appear or to become dominant when a virus is grown 

 in an unnatural host, it is possible that the altered environment of this 

 host provides a somewhat different supply of amino acids and enzyme 

 systems, and in the effort to adhere to some basic pattern it becomes 

 necessary to build into the virus structure amino acids that would not 

 be used normally. The drive to follow a basic pattern and the aberra- 

 tions that result bear a certain kinship to the forces of heredity. As a 

 matter of fact, there is a striking similarity between the properties of 

 viruses and those that have been ascribed to genes. Both may be re- 

 garded as large nucleoprotein structures that have the ability to 

 perpetuate themselves within, and only within, certain specific living 

 cells. Both can undergo sudden changes, apparently cither spon- 

 taneously or as a result of external factors and those changes are then 

 reproduced in subsequent generations. Within limits, the concen- 

 tration of both in cells can be changed by proper treatment of the host. 

 Some viruses appear to be concentrated in the cytoplasm of cells and 

 others in the nuclei. 



The similarity between viruses and genes may not be without 

 significance, for the abode of genes is the cell and no virus has been 



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