134 



CELL HEREDITY 



FIGURE 5.13. The U-tube 

 used to show that cell contact 

 is required for bacterial re- 

 combination. 



Sintered glass disc which 



allows passage of medium 



but not of bacteria 



form cells with a capsule of type b, sometimes yielding ah transformants 

 with both antigens in their capsules. The association of the two DNA's 

 is usually retained so that the DNA extracted from the doubly encapsu- 

 lated types sometimes gives linked transformations; but the coupled 

 marker genes may also separate so that cells are transformed for one 

 or the other. 



MATING IN BACTERIA 



Some bacteria, such as E. coli, may exchange their genes by a mechan- 

 ism which involves cell contact. Many different lines of investigation 

 show that this mechanism does not involve either virus or free nucleic 

 acid molecules; no filterable agent is necessary. For example, a U-tube 

 is used with a bacterial filter at the base separating the two arms (Figure 

 5.13). In each of these arms is a suspension of E. coli of a different 

 genotype. By alternatelv applying pressure to the air above the suspen- 

 sion in each arm, the liquid is forced through the filter in both directions 

 and mixed. But the bacteria cannot pass through, and each type re- 

 mains separated. Under these conditions, recombination is never ob- 

 served. It only occurs when the two strains are in the same suspension 



