THE FREQUENCY DISTRIBUTION OF SPONTANEOUS 



BACTERIOPHAGE MUTANTS AS EVIDENCE FOR 



THE EXPONENTIAL RATE OF PHAGE REPRODUCTION' 



S. E. Luria 



University of Illinois, Urbana, Illinois 



The phage geneticist is faced with the task of constructing a satisfactory model 

 of phage reproduction, in the absence of direct morphological evidence similar to 

 the one available to the macro-geneticist. Cell division, mitosis, meiosis, fertiliza- 

 tion have a solid basis of morphological observation that the modern geneticist 

 takes for granted. The virologist, on the other hand, begins where the cyto- 

 geneticist ends; in a sense, he deals directly with the units of genetic material 

 whose existence the macro-geneticists (including the bacterial geneticists) must 

 infer. Here lies his weakness, since little is known of the performance of such 

 units — and also his strength, since he can manipulate this subcytological world. 

 He is not limited to dealing with integrated units of reproduction at the cellular 

 level, but can control to a certain extent what goes into his cells. Because of this, 

 virology's methods may lead more directly to solving the problem of the mode of 

 repHcation of genetic material. 



Penetration of one phage particle into a susceptible bacterium leads to pro- 

 duction of a large number of similar particles. The intervening steps are un- 

 known. We conjecture a reorganization of the viral material, because of its 

 nonrecoverability in infectious form early after penetration (Doermann, 1948). 

 We conjecture an integration of the viral material into the cell machinery at the 

 genetic level (Luria and Human, 1950), because cell syntheses are redirected 

 toward the production of virus specific substances. The genetic complexities of 

 bacteriophage tell us that the viral specificities to be replicated are multiple 

 (Hershey, 1946b) and that the new virus may receive imprints from more than 

 one viral ancestor within the same cell (Delbriick and Bailey, 1946; Hershey, 

 1946b). There is evidence suggesting a discrete and regularly assorted nature of 

 the material determinants of these specificities (Hershey and Rotman, 1948). 



lAided by grants from the American Cancer Society (upon recommendation of the Committee 

 on Growth) and from the Research Board, University of Illinois. The competent assistance of 

 Miss Martha R. Sheek is gratefully acknowledged. 



Reprinted by permission of the author and the Long Island 



Biological Association from Cold Spring Harbor Symposia on 



Quantitative Biology, 16, 463-470 (1951). 



139 



