10 



VIRUSES 



TABLE II 



AROMATIC AMINO ACIDS AJTD PHOSPHORUS IN STRAINS OP TOBACCO 

 MO SAIC VIRUS AND IN CUCUMBER VIRUSES 1 and » 



Virus 



No. of 

 prepara- 

 tions 



"Tyrosine 



Tobacco mosaic 12 



Yellow aucuba 3 



Oreen aucuba 2 



Holmes' ribgrass.... 4 



Holmes' masked 2 



J14D1 2 



Cuciimber virus 4 ... 7 



Cucumber virus 3 ... 1 



per cent 

 3.8 

 3.9 



6.4 

 3.9 



3.8 



4.0 



Trypto- 

 phane 



per cent 



4.2 



4.2 



3.5 

 4.3 

 4.4 

 1.4 

 1.5 



Phenyla- 

 lanine 



per cent 

 6.0 



6.3 

 6.1 



6.1 



6.1 



10.2 



10.0 



per cent 

 0.56 

 0.52 

 0.54 



0.53 

 0.54 



0.55 

 0.54 

 0.56 



It can be seen that the very building blocks of these two proteins are arranged 

 in different proportions. This constitutes, then, a clear cut case in which 

 slight but definite differences in the biological activity of two strains of 

 tobacco mosaic virus have their counterpart in slightly different chemical com- 

 positions of the virus proteins. 



Thus far we have stayed pretty close to facts. Let us now permit our 

 minds to wander a bit in order to picture the place of the viruses in our sys- 

 tem of thinking. Are they matters vegetable, animal or mineral'' A moment's 

 reflection will lead to the realization that these materials stand in a unique 

 position. On the one hand, they possess the properties of chemical compounds, 

 materials much like ordinary egg albumin; and on the other hand, they are able 

 to reproduce and even to mutate within the cells of living hosts and to cause 

 Injury to those hosts, simulating, in those respects, the behavior of patho- 

 genic living organisms. They stand in the border zone between substances which 

 are living and those which are non-living. Some people feel that viruses are 

 simply chemical compounds, and they explain the ability to reproduce in the cells 

 of a living host as being due to an auto-catalytic synthesis from raw materials 

 present in the host cell, i.e., one virus particle introduced into a cell simply 

 catalyzes the production of others like itself. Other theorists insist that 

 the viruses are merely highly specialized living organisms whose material sub- 

 stances exhibit a few unusual chemical and physical properties. These two 

 points of view are the extremes; many grades of opinion Intermediate between 

 them have been advanced. The truth of the matter is that there are no facts 

 available which allow us to decide finally between the theories just advanced. 

 Suffice it to say that, in a test tube, these materials behave not like living 

 organisms but like chemical compounds. In a test tube, the viruses do not 

 undergo respiration nor any other changes which cannot be accounted for in 

 terms of simple chemical and physical laws. 



Does the hypothesis that a virus is a protein molecule actually fit all 

 of the facts? i'irst, it is necessary to define the word molecule. The chemist 

 defines a molecule as the smallest portion of matter that can exist in a free 

 state, or as the smallest particle matter can be divided into without changing 

 its chemical properties. Such a definition presupposes that all of the mole- 

 cules of a given pure substance have identical chemical properties and are made 

 up of the sEune number of atoms arranged in the same way. If, then, one is to 

 prove that viruses are protein molecules, one must demonstrate that viruses are 

 particles which cannot be subdivided without changing the chemical and bio- 



