The Episome F 



319 



terial virus particles (tadpole-shaped ob- 

 jects) adsorbed on its surface; the F - cell 

 does not, since it is genetically resistant to 

 this virus. The cytoplasmic bridge between 

 the conjugants is clearly shown. When ex- 

 conjugants of such visibly marked pairs of 

 Hfr and F~ cells are isolated by micro- 

 manipulation and are cultured, only the 

 clones from the F~ partner yield recom- 

 binants. Note, in passing, that these find- 

 ings again demonstrate how genetics and 

 cytology have aided each other's advance- 

 ment. 



In studies of Hfr by F~ crosses, it is com- 

 monly found that most of the unselected 

 markers in recombinant progeny are those 

 derived from the F~ parent. This result 

 may be explained either by the transfer of 

 the entire genome of the male to the female 

 followed by the integration of only a portion 

 of it, or by the transfer of only a portion of 

 the male genome and its integration in toto, 

 or by a combination of the two. Experi- 

 ments have been designed to test one or 

 more of these possible explanations/' 



Particular strains of Hfr and F~, both 

 marked with suitable genetic factors, are 

 grown separately and then mixed in the pro- 

 portion of 1 : 20, respectively, to assure rapid 

 contact of all Hfr with F~ cells. At various 

 time intervals, ending roughly two hours 

 after mixing, samples are withdrawn and 

 subjected to a strong shearing force in a 

 Waring blendor. This treatment is a very 

 efficient means of separating bacteria in the 

 act of conjugation and does not affect either 

 the viability of the bacteria, their ability to 

 undergo recombination, or the expression of 

 the various chromosomal genotypes under 

 test. Once separated, the bacteria are plated 

 and scored for male markers which have 

 integrated. As expected, zero minutes after 

 mixing no recombinants for male markers 



:; The following discussion is based principally 

 upon work of E. L. Wollman and F. Jacob (see 

 references at the end of this chapter). 



MINUTES RECOMBINANTS HAVING Hfr MARKERS 



None 



8 T 

 8'A T, L 



9 T, L, Az 



11 T, L, Az, T, 



18 T, L, Az, T, , Lac 



25 T, L, Az, T, , Lac, Gal 



figure 24-2. Recombinants obtained when 

 conjugation is artificially interrupted at various 

 times after mixing F~ and Hfr strains. The 

 Hfr strain has markers for T, L, Az, 7\, Lac. 

 Gal. (After W . Hayes.) 



are obtained; to obtain recombinants for all 

 the male markers, conjugation must last for 

 about 107 minutes. The time at which dif- 

 ferent male markers enter the female cell, 

 however, varies widely within this time in- 

 terval (Figure 24-2). For example, T and 

 L markers of the Hfr do not enter F~ until 

 after about 8.5 minutes of conjugation, 

 whereas the Gal marker (for galactose) re- 

 quires about 25 minutes of conjugation be- 

 fore it is transferred. T and L are known 

 to be close together and widely separated 

 from Gal in the recombination map of F + 

 (p. 314); consequently, a definite relation- 

 ship exists between time of transfer from 

 Hfr to F" and the location of the marker 

 on the Hfr chromosome. 



Had different portions of the Hfr chro- 

 mosome entered the F - cell at random 

 times, the above results would not have been 

 obtained. It can be concluded, therefore, 

 that the Hfr chromosome is transferred in a 

 specific manner: one particular end of the 

 DNA string usually enters the F - cell first, 

 since the loci that transfer do so in a regular 

 linear procession (Figure 24-2). Other ex- 



