216 BACTERIOPHAGES 



investigation might show that the breakdown of both primary and 

 secondary infecting phage is the resuk of a similar failure at some 

 early step in the infectious process, differing only with respect to 

 the proportion of particles involved. In general, some particles 

 are excluded from participation in viral growth by failure to in- 

 ject, and some by subsequent unspecified failures, under all con- 

 ditions (Chapter XVII). Genetic exclusion of superinfecting 

 phage is thus the consequence of two distinct but probably re- 

 lated kinds of physical exclusion, one measurable by the Blendor 

 experiment, the other by the breakdown effect. Both may be re- 

 garded as consequences of a pathological exaggeration of a nor- 

 mally inefficient excluding mechanism. 



3. Material Transfer from Infecting Phage to Progeny 



An interesting aspect of the fate of the infecting phage particle 

 is the transfer of its chemical substance to the phage progeny. 

 One may propose such questions as: Does the DNA of the in- 

 fecting phage particle or a major portion of it appear as a unit in 

 the phage progeny? Is the entire phage particle expendable 

 once it has served its function as a pattern for the synthesis of 

 new phage? Answers to these questions have been sought by 

 the use of isotopic labels with a detailed analysis of the distribu- 

 tion of the isotope in the various fractions of the lysate. The 

 most interesting information has been derived from experiments 

 in which the infecting phage has been labeled with different iso- 

 topes so that differential transfer of the various parts of the in- 

 fecting phage can be detected. This type of experiment has 

 already yielded much information which in relation to genetic 

 experiments has suggested mechanisms of phage replication. 



a. Lack of Transfer of Phage Protein to Progeny 



As discussed previously, the protein membranes of the infecting 

 phage particles do not participate in the intracellular replication 

 of phage. Once penetration of the phage nucleic acid into the 

 host cell is accomplished, the membranes can be detached from 



