THE CHEMICAL BASIS OF HEREDITY 



15 



frequency of transformation which they obtained was at first only about 

 one per miUion cells treated. 



A typical type transformation experiment is diagrammed in Figure 1.5. 

 Nonencapsulated cells (derived from capsular Type II) are mixed with an 

 extract from encapsulated cells of Type III, and the progeny which re- 

 sult are composed of two classes of cells: most of them are nonencapsu- 

 lated, like the parental strain, but a few, about one per million, are 

 encapsulated and of Type III, like the strain from which the extract 

 was made. This result represents the in vitro confirmation of Griffith's 

 original finding. 



When the transformed cells were isolated and grown, not only did they 

 remain encapsulated but extracts prepared from them had the same 

 transforming ability as did extracts obtained from the original en- 

 capsulated strain. It was thereby demonstrated that in vitro trans- 

 formation resulted in a hereditary change just as had occurred in vivo. 



Using this experimental procedure as a biological assay for the trans- 

 forming activity of various extracts from donor cells, Avery, McLeod, 

 and McCarty were able to isolate and purify a single class of molecules, 

 which they called transforming principle. They found that it con- 

 sisted of deoxyribonucleic acid (DNA). Further purification of the 

 extract and many analyses by other investigators have only confirmed and 

 strengthened this conclusion. Although it cannot be shown that any 

 extract is completely pure, there is no reason to believe that the activity 

 of transforming extracts results from the presence of a trace contaminant. 

 On the contrary, the behavior of the transforming extract coincides in 



eoe 



+ 



elee 



e ee j_ 



e 



strain Rll 



noncapsulated 



derived from SII 



Extract of DNA 



from SIII 



encapsulated 



cells of Type III 



Many cells 

 like Rll 



A few 



encapsulated 



cells of Type III 



like SIII source 



of the DNA 



FIGURE 1.5. Transformation of capsular type in pneumococcus. Cells of strain Rll, 

 which are noncapsulated but derived from encapsulated strain SII are treated with 

 an extract of DNA from encapsulated cells of strain SIII. After treatment, many cells 

 are recovered like the parental strain Rll but, in addition, some encapsulated cells of 

 Type III are found like strain SIII which was the source of the DNA. These cells ore 

 transformants, having acquired from the DNA extract new genetic material for mak- 

 ing Type III capsules. 



