THE CHEMICAL BASIS OF HEREDITY 



23 



Phage DNA can be made radioactive by infecting bacteria grown on 

 media containing [P'"04l~. The radioactive phosphate assimilated into 

 the bacteria is passed on to the newly forming progeny phages. Phage 

 proteins do not contain phosphorous, so only the DNA is labeled. Sim- 

 ilarly, phage protein can be selectively labeled with radioactive sulphur, 

 S^^, by growing the host bacteria on [S 04]°", since DNA does not con- 

 tain sulphur. This method of differential labeling makes it possible to 

 distinguish between phage DNA and phage protein after infection. 

 Normally after infection one finds both labels associated with the bac- 

 teria. However, Hershey and Chase, at the Carnegie Laboratory of 

 Genetics at Cold Spring Haibor, found that, if the bacteria were agitated 

 in a Waring Blendor immediately after infection, most of the P^^ label 

 was associated with the cells, and the S was largely in the surrounding 

 medium. The interpretation of these results is diagrammed in Figure 

 1.9. They postulated that, immediately after attachment of the phage to 



X Units of 



radioactivity 



(p32j 



Waring 

 Blendor 



X Units of 

 radioactivity 



x-y Units of 

 radioactivity 



<} 



y Units of 

 radioactivity 

 {y is a very small quantity) 



w Units of 

 radioactivity 



Waring 

 Blendor 



/ 



w Units of 

 radioactivity 



w-z Units of 

 radioactivity 





z Units of 

 radioactivity 



(2 is almost equal to w) 



FIGURE 1 .9. Demonstration that only phage DNA enters the bacterial host cell after 

 infection. Cells ore infected with P*~ -labeled phage, and after being allowed time 

 for infection, they are agitated in a Waring Blendor which shears oflf the phage coats. 

 Radioactivity is measured at each step in the procedure. Very little of the original 

 P'" radioactivity is lost from the cells. Thus, the phage DNA has been incorporated 

 into the host cells. When phage are labeled with S *"* in their proteins, and the same 

 experiment performed, the results are very diflFerent. Most of the radioactivity is 

 found in the supernatant with the phage protein; very little enters the cells. 



