RECOMBINATION ANALYSIS IN MICROBL\L SYSTEMS 59 



may be, or perhaps always are, unequal. In the case of bacterial re- 

 combination, the male parent may occasionally contribute a complete 

 genome, but more often it does not. In transformation, the DNA donor 

 contributes one molecule, or about 1/200 of the bacterial genome. In 

 transduction, in wliich the phage vector carries DNA of bacterial origin 

 in place of some or all of its own DNA, the degree of participation of 

 the donor parent is more often than not still smaller than in transfor- 

 mation. Let us see now what recombination looks like in transforma- 

 tion, where the molecular characteristics of the donor cell's contribu- 

 tion are fairly well defined, and where the absence of viral genome 

 offers a somewhat simpler situation. 



In the first place, genetically active DNA does not multiply as such 

 in the bacteria that absorb it. This can be inferred from the experiments 

 of Ravin ( 1954 ) , who studied the clonal distribution of transformants 

 in an instance where two distinct kinds of recombinants are formed 

 after uptake of a single kind of DNA molecule. His data show that 

 either one or the other kind of transformation occurs within the clone 

 formed by a cell which has picked up the molecule, but not both. 

 There is, thus, a unique event which gives rise to a single recombinant 

 genome, a result which would be impossible if the donor molecule rep- 

 hcated prior to recombining. This result is confirmed by recent experi- 

 ments in which the donor markers are titered at different intervals 

 after DNA uptake by the recipient cells; the measurements are made 

 by breaking open the latter, extracting the total DNA of the cells, and 

 titering it on an appropriate detector strain (Voll and Goodgal, 

 1961). 



In the second place, it is established that the recombination event 

 in transformation may involve the transfer of only part of the specificity 

 of the inducing molecule to a recombinant bacterial genome ( Ephrussi- 

 Taylor, 1951), and, indeed, that partial transfer is the rule rather than 

 the exception (Hotchkiss and Marmur, 1954; Hotchkiss and Evans, 

 1958 ) . In other words, the recombinations detected as transformations 

 are due to events which are formally analogous to a double cross-over 

 involving a very small segment of the donor DNA molecule. Since tlie 

 donor molecule comprises approximately 1/200 of the donor genome, 

 this cross-over can involve only a very small segment indeed of the 

 total bacterial genome, and the region involved is thus comparable to 

 the effectively paired segments of the genetic systems described above. 

 Therefore, in transformation we should expect to observe high cross- 

 over frequencies in this region and also to find the poor additivity of 

 map distances that is observed with intragenic markers. 



In the third place, the genome of the cell that picks up a trans- 

 forming molecule is not itself transformed. A transfomned genome is 

 formed in the course of one of the cell divisions after DNA uptake. 



