EVOLUTION AND VIRUSES 



the origin of life, and for this the wealth rather than the 

 poverty of possibilities is what sets insoluble problems. 

 Certainly there is no need to assume that all have developed 

 from pathogenic organisms, which have gradually lost their 

 form and function. As we have already noted, all biological 

 systems contain nucleic acids and proteins; nothing that has 

 yet been found about the constitution of plant viruses sets them 

 apart from the nucleic acids and proteins of "normal" cells. 

 Although larger than most "normal" nucleoproteins, tobacco 

 mosaic virus, for example, seems to contain the same amino- 

 acids as occur in other, non-pathogenic, proteins. Similarly, the 

 only purines and pyrimidines yet identified also occur in other 

 nucleic acids of the ribose type. Maybe there are quantitatively 

 minor components still to be discovered, and it would be vain 

 to pretend that knowledge about the components of nucleic 

 acid is complete, but it does seem possible that the dif- 

 ferences between viruses and other nucleoproteins lie in the 

 way their component units are put together rather than in any 

 unique components. 



Normal cells have nucleic acids of two kinds, those with 

 the sugar ribose and those with deoxyribose, and viruses also 

 fall into two types, one containing ribose and the other deoxy- 

 ribose nucleic acid. The old disagreement, seemingly irrec- 

 oncilable, between those who called viruses organisms and 

 those who called them molecules, posed an unreal antithesis. 

 Both sides overlooked the fact that there are many cellular 

 components other than viruses that do not fall neatly into 

 either category; chromosomes and cytoplasmic nucleoproteins 

 are examples containing the two types of nucleic acid and of 

 systems that also resemble viruses in seeming to reproduce 

 themselves in the right environments. These provide the closest 

 analogies for viruses and it is from such complex cellular 

 components that is it most reasonable to assume that viruses 

 have evolved. 



There is nothing novel in this idea. Muller suggested nearly 

 40 years ago that viruses were genes that had become freed 

 from the confines of chromosomes and were acting independ- 

 ently of other genes. Now, when there is information about 

 the chemical composition of viruses, his insight in relating 

 them to genetical systems rather than to pathogenic micro- 



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