A. D. HERSHEY 



kinetically, to be smaller than the pool of precursor DNA meas- 

 ured directly. The fact that no such discrepancy is found 

 means that the time precursor DNA spends in the maturation 

 cycle is short compared to the washout time of the pool (about 

 8 minutes). However, since the methods are far from precise, 

 and viral growth in different cells is not synchronized, only ex- 

 treme alternatives can be excluded; for example, linear multi- 

 plication such that each offspring particle enters its maturation 

 cycle directly at birth. To this extent the chemical findings 

 suggest a geometric mechanism of synthesis of DNA. More to 

 the point, however, is the likelihood that the amount of precursor 

 DNA determined analytically should exceed appreciably the 

 amount contained in the genetically defined vegetative struc- 

 tures. 



The chemical facts show that the infected cell contains 50 

 to 100 phage equivalents of viral precursor DNA, much of 

 which belongs to a unitary pool in the sense that it is sampled at 

 random for the maturation of infective particles. This pool is 

 largely filled 10 minutes after infection, at the time maturation 

 begins. 



What are the characteristics of the genetic pool? Luria's 

 (43) analysis of viral mutation showed that genetic deter- 

 minants increase geometrically; this is probably equivalent to 

 saying that they share a pool from which samples are drawn at 

 random during maturation. This was shown more directly by 

 Visconti and Garen (61), who found that genetic markers con- 

 tributed by superinfecting phage, entering the bacterium 7 or 8 

 minutes after primary infection, were sampled from the same 

 pool with markers derived from the primary infection. More- 

 over, the kinetics of genetic recombination suggest that the 

 mating pool has already reached its maximal size, estimated at 

 30 vegetative particles per bacterium, at the time maturation 

 begins (41). Thus the genetic pool and the viral precursor DNA 

 pool are similar with respect to unity, size, and time of formation. 

 It is a reasonable though not quite necessary inference that the 

 precursor DNA is contained in the genetic pool. 



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