68 



S. BENZER 



[vol. G3 



of the phages upon the observed survival cun^e will be negligible, since at 

 15 minutes the complex has a much higher survival than have several free phages. 

 By reference to figure 1 it can be seen that even a cluster of 100 phages would be 

 inactivated more rapidly than the 15 minute complexes. 



Experiments with T7. A complete set of survival curves for monocomplexes 

 of T7 and strain B irradiated at intervals during the latent period (12 minutes) 

 is given in figure 7. It will be observed that the resistance remains unchanged 

 (at the same value as for free phage) for 3 minutes in the presence of nutrient. 



200 

 SECONDS 



300 



100 



UV DOSE IN 



Figure 7 . Survival curves for T7 complexes irradiated at different times during the 

 latent period. Each curve is marked with the time in minutes after adding broth to the 

 complexes formed in buffer. All data are from the same growth experiment at 37 C (latent 

 period = 12 minutes). Average multiplicity of infection = 2 X 10~'. 



As time goes on, the curves become multiple-target in character, the average 

 multiplicity continually increasing, while the final slope changes only slightly. 

 This result stands in marked contrast to T2r and resembles that predicted by 

 target theory. The exact shapes of these curves are not consistent with an as- 

 sumption of equal multiplicities for all complexes at a particular time. However, 

 the shape may be explained by a distribution of the number of targets, which 

 may be expected as a consequence of different rates of phage multiplication in 

 different cells. It is well known (Delbriick, 1945) that the number of phage 

 particles released by lysis of individual cells has an extremely wide distribution. 



307 



