A. D. HERSHEY 



any method used to break open the infected cell might alter im- 

 portant structural relations. 



Direct microscopy of infected cells (50) has not revealed any 

 characteristic intracellular structures that might be vegetative 

 phage particles. Similarly, examination of extracts of infected 

 bacteria (40) brought to light only normal cell components, 

 resting phage particles, and empty, tailless membranes that have 

 been interpreted as early products of the maturation cycle. 

 Watanabe, Stent, and Schachman (63) labeled viral precursor 

 DNA by infecting cells with P^^-containing phage, and then sub- 

 jected extracts prepared at various times to centrifugal analysis. 

 Citrate was used to inhibit bacterial deoxyribonculease, but 

 otherwise the conditions of lysis may have been favorable for 

 enzymatic degradation. They found that most of the viral 

 precursor DNA in the lysates sedimented like free nucleate. 



None of these results either contradicts or justifies the as- 

 sumption that vegetative T2 consists of free molecules of DNA. 



VIRAL PRECURSOR DNA 



Early tracer experiments showed that the phosphorus used 

 to make viral progeny comes from three distinct sources: 

 materials assimilated by the bacterium before infection (8), 

 materials assimilated after infection (8), and parental materials 

 (53). Weed and Cohen (66), Stent and Maal0e (58), and 

 French et al. (18) found that atoms derived from any of these 

 three sources entered unequally into the viral progeny formed at 

 successive times, suggesting the existence of a precursor pool 

 containing a limited amount of phosphorus. These facts 

 prompted a search for viral precursor DNA, which was greatly 

 facilitated by the interim discovery that the DNA of T2 contains 

 the distinctive constituent 5-hydroxymethyl cytosine (69). 

 This search brought to light the following information. 



Cytosineless DNA, that is, DNA having the composition of 

 the DNA of viral particles, begins to form in infected bacteria 

 promptly after infection, and already measures roughly 50 phage 



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