THE CHEMICAL BASIS OF HEREDITY 19 



The frequency of doubles was found to be approximately the product of 

 the frequencies of the two classes of singles, indicating that the two 

 DNA's are incorporated independently. They may therefore be con- 

 sidered separate genetic units and, in a formal sense, separate particles. 

 Further evidence of the particulate nature of hereditary material comes 

 from studies of the reversibility of transformation. 



Reversibility of transformation 



Independence of different genetic units in transformation is found 

 frequently but not in all instances. Examples have been observed of 

 linked transformation, that is, the recovery of double transformants 

 more frequently than the product of the frequencies of the singles. This 

 example is important both because it demonstrates linkage and also be- 

 cause it provides a method of demonstrating an important principle, 

 namely, the reversibility of transformation. 



Some pneumococci cannot utilize the sugar, mannitol, for growth, but 

 mannitol-fermenting strains can be isolated. A streptomycin-resistant 

 (str-r), mannitol-fermenting {mtt') strain was prepared by serial trans- 

 formation, and its DNA was applied to a streptomycin-sensitive (str-s), 

 mannitol-nonfermenting (mtr) strain. The results, listed in Table 1.4, 

 show that the frequency of double transformants was much higher than 

 the product of single transformations for each factor. This linkage 

 makes it possible to investigate the question whether DNA from str-s 

 cells will transform resistant cells back to sensitivity, and whether DNA 

 from mtr cells will transform mtt' cells back to the mtr condition. 



' The low frequency of transformation makes mandatory the use of some 

 seli^tive device to recover the transformed cells from the great excess of 

 nontransformed ones. In transformation to drug resistance or to utiliza- 

 tioh of a particular compound for growth, the selective device is simply 

 thife ii^ of media containing the drug or the particular growth compound. 

 Ift^wfe' reverse transformation, the experimental technique employs 



TABLE 1.4 

 Double Transformation with Linked Markers 



(After Hotchkiss, 1954, Symposium on Genetic Recombination, 

 /. Cell Comp. Physiol., 45:1) 



r^ T^»,. r, ^11 Transformants 



Donor DNA Receptor Cells ^. — i p; — ri 



Single Double 



mtl^ str-r mtr str-s mtl^ str-s 0.01 



mtt str-r 0.001-0.002 

 mtr str-r 0.01 



