132 



THE CELL AND PROTOPLASM 



materials. The viruses vary in composition 

 from the amino and nucleic acids of to- 

 bacco-mosaic and tobacco-ring-spot viruses 

 to the amino acids, nucleic acid, carbohy- 

 drate, and lipoid that go to make up vac- 

 cine virus. Despite the variation in compo- 

 sition and size, there is no reason to believe 

 that viruses differ among themselves in any 

 fundamental respect, or that there is other 

 than a continuity of structure from small 

 to large viruses. All viruses have in com- 

 mon the ability to reproduce or multiply 

 when placed within susceptible living cells, 

 and since no virus has been found to mul- 

 tiply under any other conditions, they may 

 be considered obligate parasites. The man- 

 ner in which viruses multiply has been, and 

 remains, a matter of much conjecture. In 

 the case of phage, Northrop considers that 

 multiplication can be more simply ex- 

 plained by analogy with the autocatalytic 

 formation of pepsin and trypsin than by 

 analogy with the far more complicated sys- 

 tem of living organisms. This seems a 

 reasonable attitude, especially in view of 

 Krueger's evidence for the existence of an 

 inactive precursor or pro-phage. However, 

 many difficulties become apparent when 

 this viewpoint is adopted for viruses in 

 general, for no evidence has been obtained 

 for the existence of inactive precursors 

 having chemical properties similar to those 

 of the viruses, no virus has been produced 

 de novo or in the absence of living cells, 

 and a multiplicity of precursors must be 

 postulated, since a given host cell is capable 

 of producing any one of hundreds of differ- 

 ent viruses or virus strains. 



Nevertheless, it is quite possible that the 

 basic idea of catalysis may be correct with 

 respect to virus activity. Bergmann has 

 shown experimentally that a catalyst, a 

 protein enzyme, can cause the formation or 

 synthesis of a peptide linkage. There is 

 every reason to believe that proteins are 

 produced through the formation of peptide 

 linkages ; hence, it is but a step to consider 

 that proteins may result from the catalytic 

 action of still other proteins, and but an- 

 other step to consider that a protein may 

 catalyze reactions resulting in the forma- 



tion of replicas of itself. The latter pro- 

 tein, catalyzing such reactions within a cell, 

 would conform, of course, to our definition 

 of a virus. The virus reaction may, there- 

 fore, resemble the pepsin and trj^psin acti- 

 vation reactions and the reaction postu- 

 lated for phage, except that the virus 

 reaction is far more complicated, requiring 

 not one but a series of reactions and special 

 conditions which so far have not been re- 

 produced outside of a cell. 



The introduction of a virus most cer- 

 tainly diverts the normal metabolic activity 

 of a cell, yet the influence of the virus 

 might be likened to that of agents already 

 present which direct normal metabolism, 

 except that the virus exerts a dominating 

 influence. The mechanism by means of 

 which virus is synthesized within a diseased 

 cell must be very similar to that by means 

 of which normal proteins and constituents 

 are synthesized within a normal cell. 

 Levaditi has suggested that in the case of 

 a virus-infected cell the "constructive" 

 factor of the virus superimposes itself and 

 dominates the normal factor. Similar ideas 

 have been advanced in the writings of other 

 workers, especially when cancerous cells 

 were under consideration. In some in- 

 stances the loss of a factor rather than the 

 addition of one has been postulated. One 

 great point of difference is that in the case 

 of the virus reaction the "key" to the 

 disrupted metabolism, the virus, can be 

 separated, isolated in pure form, and 

 studied apart from the system, whereas in 

 the cases of normal cells or of cancerous 

 cells similar "keys" have not been found 

 separable as yet. The ' * key or keys ' ' must 

 be contained within the chromosome and 

 probably are represented by genes or gene 

 derivatives, and therefore may be nucleo- 

 proteins. The fact that all viruses so far 

 isolated have been found to consist of : 

 nucleoprotein or to contain it may be of 

 special significance, and the possibility that 

 they may have been derived from genes 

 or nuclear material has been considered 

 from time to time by different writers. / 

 Although it is quite possible that viruses 

 may have arisen originally in such an en- 



