BACTERIOPHAGE 

 An Essay on Virus Reproduction 



S. E. LURIA 



Indiana University, Bloomington 



In a discussion of the mechanisms involved in virus reproduction, 

 it is well to start with a critical revision of concepts and definitions, 

 because some of the ideas conceived in other fields have carried into 

 virology implications not justified by the methodology of virus research. 



The term "virus" itself can be operationally defined as an exogenous 

 submicroscopic unit capable of multiplication only inside specific cells. 

 This definition gives a methodological unity to the field of virology 

 and, by leaving ambiguous two borderline fields — that of obligate 

 parasitic microbes, on the one hand, and that of protoplasmic compo- 

 nents transmissible by graft only, on the other hand — suggests some 

 of the possible natural relationships of viruses. 



The concept of reproduction requires closer scrutiny. What we 

 observe is the appearance of increased virus activity, associated with 

 an increased number of specific material particles, in a population of 

 virus-infected cells. Virus is produced by the only observable entity, 

 the virus-infected cell, and the mechanism intervening between infec- 

 tion and appearance of the new virus activity cannot be postulated by 

 analogy. In many minds the terms "reproduction" and "self-reproduc- 

 tion" are connected with the idea of increase in size followed by divi- 

 sion. Closer scrutiny reveals that increase in size followed by division 

 is bound to be an epiphenomenon of some critical event of reproduction, 

 which must involve point-to-point replication of some elementary 

 structures responsible for the conservation of specificity from genera- 

 tion to generation. Thus, in dealing with cell growth and division we 

 trace the critical event to gene and chromosome dupHcation. Even a 

 bag of enzymes could only grow and multiply by duplication of dis- 

 crete enz}ane molecules, which can hardly be supposed to grow indi- 

 vidually in size and then split. In a repeat, crystal-like structure, such 

 as has been suggested for rod-shaped particles of plant viruses (2), 

 the elementary repeated unit must be replicated. In other words, all 

 growth and reproduction should ultimately be traceable to replication 

 of specific chemical configurations by an essentially discontinuous ap- 

 pearance of discrete replicas. 



One of the first tasks in virus research is to uncover the relation of 

 the virus particle, as we know it in the extracellular state, to what is 

 replicated inside the infected host. Misunderstandings may arise, how- 

 ever, if we fail to distinguish between replication and the more general 

 category of chemical synthesis. There is something peculiar to ho- 

 mologous replication that sets it aside from other types of synthetic re- 



