58 MOLECULES, VIRUSES, AND BACTERLV 



mental results with a single, unified theory of crossing-over. Further- 

 more, diflFerent degrees of stability of the initial phase of efiFective pair- 

 ing could account for the very diflFerent frequencies with which non- 

 reciprocal recombination occurs in different organisms. 



It is evident from the discussion above that recombination is inti- 

 mately associated with chromosome duplication in our thoughts about 

 the subject today. On the other hand, chromosome duplication appears 

 to be primarily a problem of DNA replication (Taylor, 1958). Very 

 interesting models of recombination have been constructed, taking into 

 account what is at present known of the mechanism of DNA replica- 

 tion (Taylor, 1958; Freese, 1958). However, we cannot enter into the 

 details of these models here, except to remark that they involve critical 

 assumptions about how the DNA is organized into a cliromosome, and 

 that these will require experimental verification. 



Recombination with unequal parental participation 



The diversity of the experimental possibilities in microbial systems 

 is due not only to the factors mentioned above but also to the discovery 

 of unexpected hybridization mechanisms. The first new system to be 

 discovered was that of bacterial transformation, in which a bacterial 

 cell absorbs DNA of high molecular weight, endowed with genetic ac- 

 tivity ( Avery, MacLeod, and McCarty, 1944 ) . The second was genetic 

 recombination in E. coll, and the third was transduction (Lederberg 

 and Tatum, 1946; Zinder and Lederberg, 1952 ) . In bacterial recombina- 

 tion, two cells of a particular constitution pair, and the chromosome of 

 the cell serving as the male is injected into the cell serving as the fe- 

 male ( see the recent review of Hayes, 1960 ) . The conjugation is fragile, 

 and much of the time only a portion of the male chromosome is trans- 

 ferred before all pairing is interrupted. One obtains zygotes of varying 

 degree of partial diploidy, in which recombination occurs during the 

 course of subsequent cell division. 



Because the degree of partial diploidy is highly variable in a single 

 mating population, this system is of limited use in recombination an- 

 alysis; in addition to the statistical properties of the recombination 

 process, one must deal also with the statistical properties of the transfer 

 mechanism. Nevertheless, the two essential characteristics of chromo- 

 somes, mentioned above, have been clearly demonstrated in this sys- 

 tem (high frequency of crossing-over in small regions of the chromo- 

 some, Rothfels, 1952; multiple mutational sites in the functional gene, 

 E. Lederberg, 1958). In transduction, a virus or virus-like entity grown 

 on a cell of one genetic constitution introduces genes of the host upon 

 which it was cultivated into a subsequent host. All of these systems are 

 characterized by one feature: the genetic contributions of the parents 



