REPRODUCTION OF VIRUSES: A COMPARATIVE SURVEY 565 



In general, celJular damage appears to be an incidental manifestation of 

 virus infection, even though it is a frequent correlate of virus maturation. 

 Cellular dysfunctions of a variety of types, ranging from simple metabolic 

 alterations in some lysogenic bacteria to unrestricted cell proliferation in 

 virus-induced tumors, must be considered as the expression of the genetic 

 functions of the virus acting in integration with the host cell genome. 



These virus-controlled functions are not necessarily different from cellular 

 functions that may arise, be modified, or be suppressed by genetic changes 

 such as mutations. Once we visualize virus infection as a form of infective 

 heredity, the problem of the possible determination of apparently normal 

 functions by virus-like elements reduces itself to the problem of the potential 

 ability of various genetic elements of the ceU to behave as viruses: that is, to 

 control their own maturation into readily transferable forms. At present, 

 this problem can be defined precisely only for phages, which, by their reduc- 

 tion to chiomosome-linlied prophages, their interactions with neighbouring 

 chromosomal elements, and their mutations to defectiveness, demonstrate 

 the possible transitions between virus and chromosomal element. In bacteria, 

 at least, transformation and transduction give proof of the intrinsic trans- 

 ferability of all the genetic material as functional DNA. With other cells, 

 the occurrence of latent infections and the activation of unsuspected viruses 

 upon transfer of tissue extracts mto new hosts have repeatedly suggested the 

 possibility of a transformation of cell components into viruses. 



Specific cellular components released by growing cells may play a role in 

 growth regulation phenomena (Weiss, 1955), as well as in tissue compat- 

 ibihty (Billingham et at., 1956). It is conceivable that some of these regula- 

 tory substances may contaia nucleic acids and may be able to reproduce in 

 cells into which they gain access. 



C. Viruses and Cellular Constituents 



Our discussion has led us to consider the natural relationship of viruses to 

 constituents of normal cells. We are not stretching our imagination too far if 

 we consider the phage DNA as a transmissible fragment of bacterial DNA. 

 Some of the RNA viruses, on the other hand, may ultimately prove related 

 or homologous to ceU microsomes, which, as part of the cytoplasmic reticu- 

 lum, are probably the carriers of the coded determinants for protein synthesis 

 in the cell protoplasm (Simkin and Work, 1957). 



A detailed discussion of virus origin would be outside the scope of this 

 chapter. We may point out, however, that even if viruses are genetically 

 related to certain cell constituents, it is unjustified to expect a detailed 

 homology between the genetic structure and j)hysiological fimctions of a 

 virus and the structure and fimctions of constituents of the cells in which 



