BACTERIOPHAGE GENETICS 359 



of the number of phages adsorbed on individual cells. Ac- 

 cording to these assumptions the frequency of productive in- 

 fection is 



Z ^' [1 - (1 - e-^n'T 

 V = 



1 - (.V + \)e-^ 



where y is the fraction of multiply-infected bacteria yielding 

 phage; x is the average multiplicity of infection; r is the av- 

 erage number of lethal damages per phage; n is the number of 

 genetic units in each phage ; and k is the exact number of phages 

 absorbed to individual cells. 



Since y, x, and r are known for a given experiment, n can be 

 calculated. This equation gav-e a fairly good fit to the experi- 

 mental data for ultraviolet doses up to 10 or 20 lethal hits per 

 particle, if n was assumed to be 25 for T2, 15 for T4, and 30 

 for T6. On carrying these experiments to much higher doses 

 of radiation, Dulbecco (1952a) found two serious discrepancies 

 between the experimental data and the predictions of the 

 theory. The theory predicts the same limiting slope for the 

 dose-survival curve following single and multiple infection. 

 The observed limiting slope for multiple infection is about ^/i 

 of that predicted. The theory requires the extrapolated linear 

 portion of the survival curves to intercept the ordinate at the 

 value k"", whereas the observed intercept is a much smaller 

 value. The discrepancies suggest a kind of ultraviolet damage, 

 lethal in single infection, that can be repaired in multiple infec- 

 tion without substitution of unique phage elements. The v^alue 

 of n can not be equated to any known genetic or chemical 

 structures in the phage. 



In a brief note Cairns and Watson (1956) pointed out that the 

 discrepancy between the observed and predicted survival curves 

 could be due to neglect of the deviation from a Poisson distribu- 

 tion of absorbed phage particles expected from the unequal 

 sizes of bacterial cells. If Dulbecco's (1949a) figures for dis- 

 tribution of cell sizes were used, and a Poisson distribution of 



