222 BACTERIOPHAGES 



was 248 and the P" transfer was 29 per cent. This experiment 

 served two purposes: it demonstrated that contamination of the 

 progeny with parental P"^^, as contrasted with incorporation of 

 the isotope in the progeny, was not a significant factor in such 

 experiments. It also demonstrated that just prior to the appear- 

 ance of mature phage in the infected bacteria there is no pre- 

 cursor DNA in a sedimentable form that might simulate mature 

 phage in its behavior. 



Kinetic experiments, in which the incorporation of parental 

 phosphorus into offspring phage particles is measured at various 

 times after infection, were reported by French, Graham, Lesley, 

 and van Rooyen (1952), Watson and Maal0e (1953), and 

 Hershey (1953a). The experiments of the last-named author 

 are especially detailed. The results of subsequent experiments of 

 the same kind, embodying minor improvements in technique, are 

 illustrated in Figure 8. They show all the features discussed 

 above. Shortly after infection, the parental DNA in prema- 

 turely lysed cultures does not sediment at centrifugal speeds 

 sufficient to throw down phage particles. It remains largely in- 

 soluble in acid, howev^er. Beginning at the end of the eclipse 

 period, the parental DNA is reincorporated into phage particles, 

 most of it entering the first ones to mature. By 50 minutes after 

 infection the maximal incorporation has been achieved, al- 

 though phage particles continue to be formed after this time. 

 The observ^ed transfer is about 70 per cent. 



c. Material Transfer Without Genetic Transfer 



In previous sections of this chapter we hav^e considered the fate 

 of the substance of the infecting phage particles when these were 

 the direct ancestors of the final phage yield. We will now con- 

 sider the available evidence with respect to transfer of substance 

 from phage particles which are not the parents in a genetic sense 

 of the final phage yield. These experiments call for mixed in- 

 fection with two phages in which matters are arranged so that 

 only one of them is capable of multiplying. 



