322 



( IIM'TER 24 



after conjugation. Thus, the question of 



whether the ends of the remnant ean rejoin 

 or not in the donor cannot be answered at 

 present. It is clear that the donated chro- 

 mosome remains linear and open during the 

 process o{ conjugation. One ean. however, 

 legitimately ask whether the two ends of this 

 chromosome or chromosome segment can 

 rejoin in the recipient after conjugation has 

 terminated. Again, if one believes in synap- 

 sis as a requirement for recombination, this 

 question ean be asked only with regard to 

 those recipients which receive a complete 

 donor chromosome. In this case, since link- 

 age of the Hfr locus and point of origin is 

 rapidk re-established in such cells, the an- 

 swer is that ends do rejoin, and quickly. 



The recombination frequencies observed 

 after conjugation depend, of course, upon 

 both the frequency of a marker's penetra- 

 tion and the efficiency with which it is in- 

 tegrated. Interrupted-mating experiments 

 reveal the sequence of markers, regardless 

 of the frequency (greater than zero) with 

 which their integration occurs. Once the 

 marker sequence is known, integration effi- 

 ciency can be studied. If, for example, mat- 

 ings are permitted to continue long enough 

 so that just about all F^ cells are penetrated 

 by the marker under test, the percentage of 

 zygotes producing recombinants for that 

 marker will indicate the efficiency of integra- 

 tion. If 50% of the recipient cells show 

 integration of a transferred marker, this locus 

 has an integration efficiency of .5. One can 

 also test whether recombinants for a given 

 locus are recombinants for markers trans- 

 ferred earlier. By these and other methods, 

 the integration efficiency after penetration 

 can be determined for various markers. On 

 the average, the integration efficiency is 

 about .5 for each marker. Therefore, be- 

 cause of differences in penetration, the closer 

 a gene is to O, the greater is its overall 

 chance for integration. Recall that when 

 one strand of a double helix of donor DNA 



is broken b\ DNase activity, incorporation 



of donor DNA into the recipient DNA frac- 

 tion is unaffected but transformation rate is 

 drasticallj reduced. Accordingly, it is the- 

 oretically possible that the decreases in the 

 overall chance for integration with distance 

 from O may sometimes be associated with 

 the breakage of one strand of double-helix 

 DNA and not the other. In this event, back- 

 bone defects might affect not only chromo- 

 somal breakage (by scission of the DNA 

 double chain), but also integration (by 

 breaking only one strand of the two at any 

 given level ) . 



Although the donor DNA rate of penetra- 

 tion is approximately constant for the first 

 half of the chromosome, it is slower for the 

 second half. Since the bacterial chromo- 

 some is about 10 T nucleotide pairs long and 

 the entire chromosome is transferred in about 

 two hours, about 10"' nucleotide pairs (about 

 34 fji) are transferred in one minute at 37° C. 

 This transfer rate is slower at lower tem- 

 peratures. Taking into account variations 

 in rate of penetration and integration effi- 

 ciency, it is possible to construct, from in- 

 terruption experiments using Hfr males, a 

 genetic map of E. coli markers whose rela- 

 tive distances are expressed in minutes. 

 Such a map is shown in Figure 24-5. 



It was mentioned that F is transferred 

 from F to F - even when known chromo- 

 somal markers are not. When an inter- 

 rupted-mating experiment is performed to 

 determine the time when the F particle in 

 F+ males is transferred, it is found that F 

 is first transferred about five minutes after 

 mixing F f and F or several minutes earlier 

 than any known marker in the chromosome 

 is transferred. We, therefore, have addi- 

 tional evidence that F is extrachromosomal 

 in nature. 



As already mentioned, Hfr strains are al- 

 ways derived from F+ strains. It was also 

 noted that Hfr strains can revert to F + , 

 indicating that Hfr harbors a latent F par- 



