444 



ROLLIN D. HOTCHKISS 



-DNA 



Parent culture 



Culture 4- rare mutant, (e.g. drug resistant, etc., 1 in 10') 



Mutant culture (or stock variant, e.g. encapsulated) 



multiplication 



selective 

 environment 



extraction 



and 

 purification 



Purified mutant DNA (transforming agent) 



+ 

 Parent culture 



i 

 Culture containing transformed cells (1 in 10^ to 10') 



Fig. 1. Transformation of a bacterial culture. 



themselves. Other developments since have revealed a number of other 

 characters to be transformable in pneumococci and in other bacteria 

 species, and have also been concerned with the relation of the DNA factors 

 to normal genetic mechanisms. 



c. Chemical Nature of Bacterial Transforming Agents 



Historically, the first source of pneumococcal transforming factor was 

 heat-killed encapsulated cells, which were shown to act, first in the in- 

 fected mouse*"^ and later in the test-tube,*^ upon unencapsulated cells. 

 The ultimate recognition of the activity of a DNA^^ came after the suc- 

 cessful preparation of the agent in a cell-free form by Alloway.^^ 



««F. Griffith, J. Hyg. 27, 113 (1928). 



61 M. H. Dawson, J. Exptl. Med. 51, 123 (1930) ; M. H. Dawson and R. H. P. Sia, ibid. 



54, 681 (1931). 

 «2 J. L. Alloway, J. Exptl. Med. 56, 91 (1932); 57, 265 (1933). 



