CHEMISTRY AND VIRAL GROWTH 



idea of reactivation by genetic substitution. Their experiments 

 are of tlie following type. Bacteria are infected with two kinds of 

 phage differing by several genetic markers, one kind carrying 

 radiation damage and one not. In Stent's experiments, the 

 damages result from decay of P^^ incorporated into the virus 

 during prior growth. In Doermann's experiments they are 

 produced by irradiating phage with ultraviolet light. In both 

 instances the damage can be ascribed plausibly to localized 

 effects on the viral DNA, and both types of damage show the 

 same behavior in radiogenetic experiments. 



The following description refers specifically to Doermann's 

 experiments. He uses T4 particles irradiated with ultraviolet 

 light until they are unable individually to infect a bacterium in 

 the usual way. Such particles may be called dead, and the 

 radiochemical process producing them may be called primary 

 killing. The dead particles can be revived in several ways. In 

 what follows we are concerned with only one of them, called 

 cross-reactivation. It shows the following characteristics. 



7. Yields from individual bacteria infected with one or 

 more live plus one dead phage particle frequently show some but 

 not others of the genetic markers introduced with the dead 

 particle. The response of the individual markers to varying 

 radiation dosage corresponds to a target volume that is 4 per cent 

 of the target volume for primary killing. 



2. Markers known from genetic tests to be unlinked are in- 

 activated independently of each other. 



3. For moderate radiation dosages, markers derived from 

 the irradiated phage particles are either absent entirely, or 

 appear in numerous copies in yields from individual bacteria. 



4. Inactivation of genetically linked markers occurs in a 

 correlated manner. 



Doermann interprets these facts in the following way. 

 Most of the radiation damage results from localized photo- 

 chemical reactions in genetic material; otherwise correlated 

 inactivation of unlinked markers would be observed. Un- 

 damaged pieces of genetic material are rescued by recombina- 



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