INTRACELLULAR MULTIPLICATION OF BACTERIAL VIRUSES 267 



nonirradiated phages and is to a large part transferred into dead progeny- 

 particles and that, in transfer experiments involving mixed infection of 

 bacteria with genetically different parents, a considerable part of the parental 

 label stays "true" to the genetic marker with which it was introduced are 

 further powerful arguments against the notion of transfer by breakdown and 

 resynthesis (Hershey et al., 1954; Hershey and Burgi, 1956). 



Another conceivable interpretation of the incomplete transfer might be that 

 the phage DNA consists of two portions, one of which is transferred intact, 

 while the other is not transferred at all. This idea was tested both by Maaloe 

 and Watson (1951) and Hershey et al. (1951b), who carried the transfer 

 experiment through two successive cycles of phage growth. They reasoned 

 that if the transferred atoms represented a special "transferable" part of the 

 viral DNA, then the labeled atoms introduced by the original parents should 

 be concentrated in the transferable sector of the first generation progeny and 

 hence be transferred to the second generation progeny with much higher 

 efficiency during a second-growth cycle. In fact, the same transfer efficiency 

 was fomid in both cycles. This experiment was later extended to a third 

 generation transfer (Stent et al., 1958) with results which can be schematized 

 as follows: 



Parent (label = 100 %) -> 1st progeny (label = 50 %) -» 



2nd progeny (label = 25 %) -> 3rd generation progeny (label = 12%) 



One may conclude, therefore, that the DNA that is transferred neither 

 originates from a preferentially transferable part which remains intact nor 

 winds up preferentially in a part which is preferentially lost in the next 

 infection. It would appear, rather, that the reasons for the incompleteness 

 of the transfer must be sought among random losses experienced by the 

 entire parental DNA in the course of infection, replication, and matura- 

 tion, as well as among certain technical difficulties inherent in the transfer 

 measurements themselves (Hershey, 1956; Hershey and Burgi, 1956). Further 

 insight into the transfer processes can be obtained by breaking open bacteria 

 infected P^^-labeled phages at various stages of the latent period and 

 examining the intracellular state of aggregation of the parental DNA. Such 

 experiments show that during the eclipse most of the parental phosphorus 

 remains part of free DNA fibers, the remainder being broken down to low 

 molecular weight substances (Watanabe et al., 1954). After the termma- 

 tion of the eclipse, the parental DNA is reincorporated into the first mature 

 progeny phages making their intracellular appearance, each phage particle 

 receiving on the average 2 % of the parental atoms. Particles maturing 

 at later times receive less and less of the parental atoms, until the transfer 

 is essentially complete by the time that the average number of intracel- 

 lular progeny has reached 100 (French et al., 1952; Watson and Maaloe, 1953; 



