REPRODUCTION OF VIRUSES: A COMPARATIVE SURVEY 551 



Four main approaches provide information on virus multiplication: (1) 

 the kinetic approach, which follows the increase in numbers of virus particles 

 by measurements of infectivity or of other specific virus properties; (2) the 

 cytodiemical approach, which studies the structural changes in cell organiza- 

 tion accompanying virus production and the localization of viral materials 

 within infected cells; (3) the hiocfiemical approach, which analyzes the bio- 

 synthesis of virus constituents, their origin, fate, and continuity, and the 

 alterations in cellular functions correlated with virus multiplication; and 

 (4) the genetic approach, which traces the continuity and variation of 

 the specific determinants of virus properties, their organization within the 

 virus, and the interactions between viral and cellular determinants of 

 specificity, 



AU these approaches must be utilized, and their results correlated, in order 

 to obtain a complete picture of virus multiplication. Only for some bacterio- 

 phages has such a program of research been carried out to any great extent; 

 a number of other viruses are now being studied in similar ways. 



B. Virus as Genetic Determinant 



The results of these studies have led to what we consider as two central 

 generalizations: the concept of virus multiplication as an altered pattern of 

 biosyntheses in an otherwise functional cell; and the concept of the virus as 

 contributing to the ceU a set of genetic elements, which initiate and deter- 

 mine the new biosynthetic pattern. 



We shall first elaborate these concepts; then, outline the evidence available 

 from various areas of virology to support and specify them; and finally, 

 discuss briefly the meaning of these concepts for the interpretation of cyto- 

 morphogenetic and pathogenic effects of viruses and of the relationship 

 between viruses and cellular constituents. 



C. Virus Replication and Virus Maturation 



Virus action within the host ceU consists essentially in the production of 

 abnormal or unusual cellular products as a result of exact specifications 

 contributed by the virus itself. The unusual cell products may include virus 

 particles, virus-related materials, and also cell constituents that have no 

 obvious similarity to the component parts of the virus particles as observed 

 in the free state. Virus infection can properly be considered as a form of 

 infective heredity, in the sense that the essential contribution of the infecting 

 virus is to introduce into the infected host cell a functional material, which 

 may be only a small portion of the infectious particle, and which contains 

 the exact specifications for the unusual syntheses that will ensue. That is, the 



