260 BACTERIOPHAGES 



phage particles per bacterium in 2 hours. The total DNA in- 

 creases at the same rate as the phage starting at about 100 units 

 per bacterium at the time of infection and reaching about 700 

 units in 2 hours. The amount of bacterial DNA per cell de- 

 creases approximately linearly from between 50 and 100 units 

 at the time of infection to less than 10 units at 30 minutes. This 

 means that essentially all the bacterial DNA should have 

 entered the pools of raw materials within 30 minutes after in- 

 fection, a result in agreement with most earlier studies. The 

 phage DNA (containing HMC) has already increased to about 

 10 units per bacterium at 5 minutes after infection and thereafter 

 increases linearly at a rate of about 7 units per minute per cell. 

 From the time that the first mature phage particles appear there 

 is a constant excess of about 80 units per cell of phage DNA above 

 that incorporated into mature phage. This is the pool of phage 

 precursor DNA. 



The properties of the pool of phage-precursor nucleic acid 

 were reported by Hershey (1953a), who investigated the size of 

 the precursor pool, the rate at which raw materials from various 

 sources entered the pool, and the rate at which DNA was with- 

 drawn from the pool by phage maturation. The technique was 

 to label various source materials with P^^ and then determine the 

 rate at which this label entered the precursor pool and the ma- 

 ture phage. The various experiments involved labeling of 

 phosphorus assimilated before infection, parental phage phos- 

 phorus, phosphorus assimilated after infection, and phosphorus 

 assimilated during a few minutes only. The following conclusions 

 were drawn from this study. 



During the first 10 minutes of phage growth in T2-infected 

 E. coli a pool of DNA is built up that is later to be incorporated 

 into phage. This pool receives phosphorus from bacterial DNA 

 but does not include bacterial DNA. Ten minutes after infection 

 phage maturation starts and DNA synthesis and phage matura- 

 tion keep pace so that the amount of phage precursor DNA re- 

 mains constant. The pool of phage precursor contains 50 to 100 

 phage particle equivalents of DNA per bacterium. Neither the 



