S7AGES IN PHAGE MULTIPLICATION 181 



ments designed to explore the role of DNA during infection. He 

 infected bacteria with phage in various combinations such that 

 the parental phage DNA, the newly synthesized intrabacterial 

 DNA, or both, contained ?■'- of high specific radioactivity. 

 Phage growth was allowed to proceed for varying time intervals, 

 and samples of the infected culture were frozen. Decay of 

 radiophosphorus occurred for various times in the frozen state, 

 after which the samples were thawed and titrated to measure the 

 number of infected bacteria still capable of producing phage. 

 The results may be summarized as follows. 



/. When only the parental viral DNA contained P'^-, the 

 ability of the infected cells to produce phage was initially about as 

 sensitive to radioactive decay as are free phage particles. As 

 phage development progressed, however, this sensitivity was 

 gradually lost. By the ninth minute it had disappeared com- 

 pletely. 



2. When the newly synthesized viral DNA (and indeed all the 

 intrabacterial phosphorus excepting the parental viral DNA) 

 contained P'-, the ability of the infected cells did not at any time 

 reach a stage at which the ability to produce phage was ap- 

 preciably sensitive to radioactive decay. 



3. When all the intrabacterial DNA contained P^-, the results 

 were virtually the same as when only the parental viral DNA 

 contained P^^. 



These results show in an interesting way the dependence of 

 viral growth during its early stages on the integrity of the paren- 

 tal viral DNA. Otherwise they show the same remarkable fact 

 revealed by the experiments with ultraviolet light, namely, that 

 the T2 phage-producing mechanism becomes highly resistant to 

 damage to intrabacterial DNA near the mid-point of the latent 

 period. Stent suggests three ways in which such stabilization 

 might be brought about: (/) a stage in viral growth may be 

 reached after which damage to DNA can be repaired, as in the 

 phenomenon of multiplicity reactivation (Chapter XVIII) ; 

 (2) a stage may be reached in which some or all the intrabac- 

 terial DNA becomes insensitive to radiochemical damage; or 



