REQUIREMENTS FOR PHAGE PRODUCTION 249 



mediuin in which phage [growth occurs. Much of the work along 

 this line was done by Putnam, Kozloff, and Evans, summarized 

 in several reviews (E. A. Evans, Jr., 1952; Putnam, 1952; 

 Kozloff, 1953). 



a. Materials Derived from the Parental Phage 



The infection of one bacterial cell by a single phage particle 

 can result in the production of 100 or more progeny phage 

 particles. It is evident that quantitatively the contribution of 

 parental substance to phage progeny must be small. Nonethe- 

 less, this topic has been very actively studied because of the evi- 

 dence it may furnish about the mechanisms of phage reproduc- 

 tion. This work was discussed in detail in Chapter XIII. It is 

 sufficient here to recall that only constituents of the parental 

 DNA are significantly transferred to the offspring, with an 

 efficiency of about 50 per cent. There is no reason to suppose 

 that this transfer is obligatory in a nutritional sense, indeed, only 

 the first offspring particles to be formed contain measurable 

 amounts of parental phosphorus. In point of fact, however, the 

 transfer cannot be prevented by experimental means. 



b. Protein Materials Assimilated before and after Infection 



The contributions from these two sources can be measured 

 separately and should add up to the total phage protein. By 

 growing the bacteria in a medium appropriately labeled with 

 isotopes one can label the bacterial proteins, nucleic acids, or 

 both. Then by transferring to an unlabeled growth medium one 

 can measure the bacterial contribution to the phage substance. 

 The dilution suffered by a given isotope in any compound in 

 going from the bacterium to the phage is a measure of the bac- 

 terial contribution providing exchange reactions are ruled out. 

 Similarly by growing the phage on unlabeled bacteria in a labeled 

 growth medium one can determine the fractional contribution 

 to phage substance made by the medium. One may study the 

 process kinetically by introducing an appropriate isotopic label 



