RADIATION AND VIRUSES 



357 



Phage T2 has similarly been investigated with X rays (Latarjet, 1948). 

 The picture is simpler than with ultraviolet since the suppression curve 

 remains constant for several minutes, then becomes of a multiple-hit type 

 (with lesser ultimate slope than for free phage), and finally becomes a 

 multiple-hit curve with ultimate slope similar to that of free phage. 



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Fig. 9-2. Intracellular irradiation of phage T7. Bacteria were washed in buffer, 

 infected with phage T7 (single infection), and then placed in a nutrient medium. 

 Samples were taken at intervals, chilled, exposed to ultraviolet radiation, and then 

 plated to determine the number of bacteria that still liberate phage. The suppression 

 curves are compared with the inactivation curve for free phage. Exposure conditions 

 were the same as for Fig. 9-1. The time given for each curve is the time between 

 addition of nutrient and chilling previous to irradiation ("growth timeT). {Benzer, 

 1952.) 



Thus this type of analysis, although quite incomplete, suggests that a 

 small phage such as T7 reproduces by multiplication of uniform elements, 

 whereas T2 undergoes a complex series of changes, including a process of 

 multiplication, which only in the latest phases leads to the presence of 

 mature fully sensitive phage particles in the infected cells. This con- 

 clusion is in accord with all our information on the reproduction of these 

 viruses. 



The intracellular irradiation procedure allows the determination of the 



