132 CELL HEREDITY 



of transformants, whereas the latter only will be lowered if competition 

 occurs inside the bacterium. As Table 5.3 shows, both are inhibited 

 equally. Furthermore, when cells are exposed to a highly polymerized 

 extract of DNA marked with P^~, the tracer is found in the cell and can 

 be released again if the cell is lysed artificially soon after penetration. 

 But when such P"^" DNA is broken into smaller pieces, it does not enter. 

 This sets a minimum size for the transforming particle, but as was the 

 case with transduction, there is in practice also a maximum size, for most 

 bacterial genes are transformed independently (see Table 1.3). 



However, when genes are closely linked, they may be transformed to- 

 gether as we saw was the case with streptomycin resistance and mannitol 

 fermentation in pneumococcus (Table 1.4). We suppose that the excess 

 double transformants are a function of structural linkage in the DNA, 

 and that incorporation is actually a recombination and follows some 

 copy-choice mechanism, as in the case of transduction. 



The recombinations being discussed are, in the final analysis, events 

 occurring between DNA molecules. Although our concepts of the 

 mechanism of these events may need modification, it is clear that new 

 types of DNA are the result. Transformation experiments give the 

 sharpest evidence for this thesis. A good case is that of transformation 

 between strains of pneumococcus with different amounts of capsular 

 antigen. Type SIIl-N is a smooth, wild type, and possesses a fully 

 developed capsule; DNA extracted from it and applied to unencapsu- 

 lated, rough (R) cells yields some transformants, and all are SIII-N like 

 the donor of the DNA. In addition, there are intermediate smooth types 

 such as SIlI-2, SIIl-1, Slll-lfl, SIIl-1/?, and SIII-lc, which have smaller 



Some Interactions Between Pneumococcus Cells of Intermediate Capsule 

 Content and Their DNA's 



(From Ephrussi-Taylor, 1951, Exp. Cell. Res., 11:589) 



Treated Strain Source of DNA Results 



SIII-1 Slll-la Slll-la + SIII-2a 



Slll-la SIII-1 SUM + SlII-2a 



SIII-1 SUl-lb SUl-lh + sm-2a 



SIII-1 SIII-lc SIlI-lc + SlII-N 



SIIl-lc SUM SIII-1 + SIIl-N 



SlII-1 SIII-2 SIII-2 + SlII-N 



Slll-lfl SIII-lc SIII-lc + SIII-N 



SIII-lc Slll-la Slll-la + SIII-N 



SIlMa SIII-2 SIlI-2 + SIIl-N 



SllI-1/; SIII-2 SIIl-2 + SIll-N 



SIII-lc Sin-2 SIlI-2 + SIII-N 



