700 A 



383 



FIGURE 42-1. Diagrammatic 

 representation of the struc- 

 tures observed in intact and 

 triggered T-even phages of 

 E. coli. 



HEAD 



>TAIL 



mosaic virus and turnip yellow mosaic virus, 

 are relatively simple structures. 



We are now in a position to examine the 

 evidence concerning the chemical identity of 

 the genetic material in typical DNA-contain- 

 ing phages. Since DNA contains no S and 

 T2 phage protein contains no P, the DNA in 

 one sample of phage can be labeled by feeding 

 the E. coil host cells radioactive P^% while the 

 protein in another sample of phage can be 

 labeled by feeding the host cells radioactive 

 S^^ The two samples of radioactive phage 

 are then permitted to infect nonlabeled cells. ^ 



^ This account follows the work of A. D. Hershey 

 and M. Chase (1952). 



It is found that all of the P^'- (hence all of the 

 DNA) enters the bacterium while all but 

 about 3% of the S'*^ (hence almost all the 

 protein) remains outside. As mentioned in 

 the last Chapter (p. 374), protein portions of 

 a phage can be shaken off the host cell by 

 Blendor treatment without affecting the 

 normal outcome of infection (transduction, 

 lysis, lysogenation). This result is consistent 

 with the view that it is DNA which is the 

 carrier of phage genetic information. 



It has already been mentioned (p. 379) that 

 naked DNA does not penetrate normal E. 

 coli. It is possible to remove the cell wall of 

 E. coli by suitable culture conditions to pro- 



