20 CELL HEREDITY 



TABLE 1.5 

 Reversibility of Transformation Using Linked Markers 



(AfttT llotchkiss, 1954, Ssriiposiiim on (ictictic l^ccoiTihinafion, 

 J. Cell. Comp. rhtfsiol.. 45:1) 



,^^ ,, ,, .. Si'lcctc'd Double Transformants 



Donor DNA Keccptor CtMls ^. r , , r r o- i . 



1 ranstormaiits (as * or Singles) 



mtl'^ str-s mtr str-r 145 mtl'*' 14.5% mtl'*' str-s 



mtr str-r mtl'^ str-s 134 str-r 12.0% mtr str-r 



double transformants, as shown in Table 1.5. The experiment is set up 

 so that single transformants of one class are selected on the appropriate 

 medium, and then they are individually tested for the occurrence of the 

 second transformation. Since there is 10-20 per cent linkage in this sys- 

 tem, it is only necessary to test about five to ten times as many colonies 

 as have been doubly transformed. The results of such an experiment 

 indicate that there is about equal probability of transformation in either 

 direction. This is true for both the streptomycin and the mannitol 

 factors, and in analogous experiments with other determinants, similar 

 results have been obtained. 



In the transformation system, the method used to demonstrate un- 

 equivocally that a transformed cell has really been changed genetically 

 is to test the transforming ability of its DNA. When a cell is trans- 

 formed from streptomycin sensitivity to resistance, one may inquire 

 whether the transformed cells (and their descendants) now contain only the 

 new determinant, or whether they contain both the old and the new. 

 The reversibility experiments demonstrate that transformation involves 

 the replacement of the old determinant by the new. Furthermore, the 

 yield of double transformants, resulting from linkage, is the same re- 

 gardless of whether the change is from mtl'^ str-s to mtr str-r or from 

 mtr str-r to mtl^ str-s. This result indicates that the spatial relations 

 responsible for linkage are unaltered by the condition of the marker, that 

 is, whether it is mtl'^ or mtr, str-r or str-s. 



These experimental results lead one to view a hereditary determinant 

 or gene as part of a DNA molecule capable of assuming at least two 

 alternative states, only one of which can be present at a time. Recent 

 studies of capsular type transformation have provided evidence that 

 there are several alternative states of capsular determinants which can 

 replace one another but do not coexist. These alternative states are called 

 alleles. 



