RECOMBINATION ANALYSIS EST MICROBIAL SYSTEMS 43 



This is a serious complication for a unified molecular theory. Strik- 

 ing as they are, these calculations only demonstrate a phenomenon of 

 which geneticists have been aware for years— namely, that distances ex- 

 pressed in terms of recombination frequency are far from constant. A 

 given segment delimited by mutants AB will yield different recombina- 

 tion frequencies of A and B, depending upon the genetic composition 

 of the remainder of the parental genome. On the one hand, the less 

 closely related the two parents of a cross are, the lower, in general, 

 will be the recombination frequency at meiosis. On the other hand, if 

 an inversion is present in one chromosome, recombination frequencies 

 in independant chromosomes will be markedly increased ( Schultz and 

 Redfield, 1951). Finally, the relative distances separating a series of 

 markers may not be the same in meiotic as in mitotic recombination 

 ( Pontecorvo and Kaf er, 1958 ) . These last facts are more weighty than 

 calculations in suggesting that the Morgan unit may not be related to a 

 fixed length of nucleotide sequence. 



There are two ways of reacting to this situation. One is to consider 

 as untenable the notion that recombination in all organisms can re- 

 ceive a unifying explanation in terms qf molecular genetics. The sec- 

 ond is to consider that recombination is indeed a molecular phenome- 

 non, just as transformation and phage genetics so strongly suggest, but 

 that additional modifying phenomena intervene when DNA is organ- 

 ized into the chromosomes of higher organisms. The second attitude is 

 positive and leads to re-examination of existing data and to the per- 

 formance of new experiments. This is the attitude adopted in the 

 present discussion. 



Clearly, the first matter to settle is: What is this yardstick used by 

 geneticists? Is it long enough to measure long distances and finely 

 enough subdivided to measure short ones? Is its variability real, or is it 

 due only to our applying it in too crude a fashion? Our success in de- 

 fining various types of functional and structural units of chromosomes 

 in molecular terms will depend upon our finding a satisfactory answer 

 to these questions. This is why understanding recombination has be- 

 come the central problem of genetics today, after having been con- 

 sidered a dead subject for some 20 years. 



Let us now turn to a consideration of the new genetic data pro- 

 vided by microbial systems. First we shall trace the major changes 

 these data have dictated concerning chromosome structure and recom- 

 bination. Second, a model will be presented, developed for the special 

 case of bacterial transformation, in order to show the ultimate conse- 

 quence of current recombination theories. Although developed for 

 transformation, this model may well be applicable to all forms of 

 genetic recombination in which the distance between two markers is 

 less than a Hnear sequence of 10,000 nucleotides. 



