GENERAL ZOOLOGY 



mosaic disease of tobacco. In addition, many types of viruses are known 

 which attack the cells of bacteria; these are called "bacteriophages." 



With the development of the electron microscope (see Fig. 2.4, p. 16), 

 which makes possible the visualization of particles thousands of times smaller 

 than the smallest ones visible with the light microscope, virus particles could 

 be "seen" for the first time (Fig. 8.26). An individual particle has the 

 general characteristics of a complex molecule, and viruses can be prepared as 

 crystalline substances. In this "non-living," crystalline state they are infective 

 for living cells of suitable hosts. Infection with a virus disease involves the 

 introduction of a few particles into the cells of the host. These particles 

 rapidly monopolize the metabolic processes of the host cell, shunting its 

 synthetic mechanisms from the manufacture of the normal products of the cell 

 to the duplication of virus nucleic acids and proteins. This, of course, pro- 

 foundly affects the vitality of the host cell, and the symptoms of the disease 

 are the direct or indirect results of such interference with cellular metabolism. 

 Studies with bacteriophage have revealed that bacterial cells sensitive to the 

 "phage" may be disrupted within a few hours, releasing thousands of new 

 virus bodies. On the other hand, a virus particle entering an insensitive 

 bacterial cell may remain quiescent, behaving more or less like a normal gene 

 of the cell and being transmitted like a gene from one generation of the 

 host to another. 



The origin and possible relationships of viruses are unknown. By some, 

 they are interpreted as modern representatives of an extremely primitive stage 

 in the evolution of terrestrial life. If viruses are primitive, they may well 

 represent a stage of life at which large, complex molecules of protein and 

 nucleic acid first evolved the ability to metabolize in the presence of the 

 proper substrate, and to organize the surrounding materials into duplicates 

 of their own structure. According to this hypothesis, the metabolic require- 

 ments of viruses, once plentiful everywhere, can now be found in the proper 

 combinations only within living cells. Viruses have been called "naked 

 genes," and indeed their composition and activities, particularly the phe- 

 nomenon of self-duplication, seem to be similar to those we associate with 

 genes. All studies agree, however, in the conclusion that viruses contain no 

 DNA (see p. 22); as in no other organisms known, the mechanisms of heredity 

 in viruses appear to involve only proteins and RNA. 



Alternatively, the viruses may be regarded as extremely degenerate forms of 

 life, which in adaptation to parasitism have reached the ultimate in parasitic 

 reduction. By this interpretation, viruses have become so specialized and so 

 dependent upon conditions within host cells that when removed from this 

 environment they revert to a "non-living," crystalline state. Whatever their 

 true nature and history, viruses are clearly on the boundary line between 

 living and non-living matter. The intensity of present research into the 

 characteristics and properties of viruses is a measure of the potential value 

 of these studies in terms of the conquest of disease, as well as of the increase 

 in our knowledge of the vital processes of living things. 



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