CHEMISTRY AND VIRAL GROWTH 



equivalents per bacterium 10 minutes after infection, at which 

 time infective virus particles begin to form. Also at later times, 

 infected cultures contain more cytosineless DNA than can be ac- 

 counted for by infective particles, and the amount of surplus 

 DNA remains approximately constant at 50 to 100 phage 

 equivalents per bacterium (28). Tracer experiments (22) show 

 that this surplus DNA is the principal immediate precursor of 

 the particles. First, because phosphorus incorporated early into 

 the surplus DNA disappears from it later on, as if it were being 

 transferred to infective viral particles by an efficient, irreversible 

 process. Second, because the kinetics of the transfer, whatever 

 the source of labeled phosphorus, calls for a precursor pool con- 

 taining 50 to 100 phage equivalents of phosphorus per bacterium, 

 in agreement with direct measurements of the amount of surplus 

 DNA. This agreement shows that the precursor pool is sampled 

 more or less at random when a phage particle is formed. In 

 this sense it must contain chiefly a single precursor, as opposed to 

 a sequential series of precursors. This important distinction, 

 based on analytical methods admittedly not very exact, is re- 

 called below by the term unitary pool. 



IDENTITY OF PRECURSOR DNA AND VEGETATIVE PHAGE 



The idea of vegetativ^e phage is based on genetic experiments 

 pointing to the existence of a mating pool or, more generally, a 

 genetic precursor pool. The chemical experiments reveal a 

 pool of viral precursor DNA. These two pools can be com- 

 pared with respect to unity, size, and time of formation, and 

 should resemble each other in all these respects if vegetative 

 phage is or contains the viral precursor DNA. 



Strictly speaking, the pool of precursor DNA cannot be a 

 unitary pool, because at intermediate stages during maturation, 

 any DNA contained in the immature particles would belong 

 neither functionally to a randomly sampled pool, nor analyti- 

 cally to mature phage particles. The effect of this complication 

 would be to cause the pool of precursor phosphorus, assayed 



11 



