A. D. HERSHEY 



bacterial DNA- thymine into viral DNA-thymine (29). The 

 presence of thymidine in unlabeled culture medium during 

 bacterial growth will not suppress the conservation of labeled 

 bacterial DNA-thymine (25). In fact, the amounts of radio- 

 carbon in all the DNA-bases remain constant during 6 hours of 

 growth at one generation per hour in media containing any of a 

 number of competitive substrates. One concludes that infection 

 stimulates a decomposition of bacterial DNA that is different in 

 rate or kind from any that may occur during bacterial growth. 

 It may be added that the decomposition in infected bacteria is 

 not prevented at concentrations of magnesium too low to permit 

 the action of intracellular deoxyribonuclease as tested in another 

 way (29), so that the effect may not be due merely to the activa- 

 tion of this enzyme (35). 



Bacterial DNA is not the only source of preassimilated 

 material available for the synthesis of viral substance. There is 

 another source of purines and pyrimidines, probably RNA (29). 

 There is also a small amount of preassimilated sulfur, which may 

 or may not be derived from protein, that can be used to make 

 viral protein (29). However, these materials are used at rates 

 altogether negligible compared to the rate of conversion of 

 bacterial to viral DNA. 



It is reasonable to conclude that the preferential breakdown 

 and re-use of bacterial DNA indicates some close relationship, 

 structural or spatial or both, between the two kinds of DNA in 

 infected bacteria. 



T2 and Other Organisms 



At the present time, if one wished to invent a genetic mate- 

 rial, one would almost certainly choose a DNA constructed 

 along the lines proposed by Watson and Crick (64). A few 

 organisms have actually chosen DNA, the evidence being very 

 good for several bacteria (31), and what I have related above 

 for T2. 



These facts immediately raise two questions. The first is 



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