RECOMBINATION IN VIRUSES AND BACTERIA 141 



nique. When Hfr are labeled with P'^^, it is found that this tracer will 

 pass with the genetic material into the F~ mate, and in a reciprocal ex- 

 periment it can be shown not to pass in the other direction. Now if the 

 Hfr DNA is labeled with P'^^, and if this element is allowed to disinte- 

 grate to different extents by storage in the cold before the cross is made, 

 then the likelihood that a gene will be transferred is proportional to its 

 distance from the anterior end of the chromosome. This method is, in 

 principle, like the interrupted mating technique. The longer P^^ is 

 allowed to disintegrate, the more likely is the occurrence of a chromo- 

 some break that will prevent the transfer of genes distal to it. Genes 

 farther from the front end of the chromosome are thus more hkely to be 

 separated and left behind than more anterior genes. The relative rates of 

 loss of the individual marker genes is similar to the relative times of 

 entry of the same markers, as Table 5.7 shows. The coincidence of 

 results suggests that the P^^ is uniformlv distributed along the chromo- 

 some and that the chromosome enters the F~ at a constant rate of about 

 10^ nucleotide pairs per second. There seems little doubt about the linear 

 order of genes in the bacterial linkage group. 



There is considerable doubt, however, about the nature of the bac- 

 terial chromosome, and little information about the mechanism of ex- 

 change itself. Bacterial cells can be shown to possess microscopically 

 visible '"nuclei," but these have not been shown to divide by mitosis. 

 They have, on the other hand, some of the basic properties of chromo- 

 somes in higher forms. They are Feulgen-positive because they contain 

 DNA, and they are duplex, as shown by the experiments of Meselson and 

 Stahl (Chapter 7). Furthermore, Witkin has shown that they are prob- 

 ably the sites occupied by some bacterial genes. She found that lac~ 

 mutants induced by ultraviolet light usually appeared as whole lac' col- 

 onies, half colonies, or quarter sectors according to whether the cells 

 irradiated were preponderantly uninucleate, binucleate, or quadri- 

 nucleate. Whether the structure of these nuclei involves other constitu- 



TABLE 5.7 



Comparison between the Relative Rates of Entry of Genes on the 



Hfr Chromosome with the Relative Rates of Loss of the Same Genes after 



P^~' Disintegration 



(From Jacob and Wollman, 1958, in The Biological Replication 



of Macromolecules. New York: Academic Press, p. 75) 



thrleu azi T\ lac\ galh 



Rate of loss 0.35 0.39 0.43 0.67 1.00 



Rate of entry 034 036 044 072 LOO 



