H20 



CHAPTER 24 



periments reveal that energy is required for 

 the transfer process, and that the entrance 

 rate is uniform from the first part of the 

 chromosome to be transferred. O ( represent- 

 ing the "origin"), up to and including the 

 locus of Lac (for lactose ). 



Whether or not they receive or lose a seg- 

 ment of an artificially ruptured chromosome, 

 both the female and male cells are able to 

 sun ive. Chromosome rupture may also oc- 

 cur spontaneously. Because of such break- 

 age, the spontaneous transfer of the Hfr 

 chromosome is usually partial. Conse- 

 quently, a piece of chromosome, called a 

 merogenote, of variable size is sent into the 

 recipient cell. As a resut. the zygote of an 

 Hfr cross is a partial diploid — a merozygote 

 — produced by a process of partial genetic 

 exchange, meromixis. These three new 

 terms are applicable also in transformation. 



The frequency of recombination is rela- 

 tively low for all chromosomal genes in an 

 F^ strain. In Hfr, however, this frequency 

 is high for those markers nearest O, although 

 it decreases as the distance of the markers 

 from O increases, and is only 0.001-0.01% 

 for the markers furthest from O. As men- 

 tioned previously, only rarely is an offspring 

 of Hfr by F- itself Hfr. An Hfr offspring 

 arises only when the marker furthest from 

 O — the terminal marker — has also been 

 transferred. This fact leads us to believe 

 that the locus responsible for Hfr is located 

 in the chromosome. Moreover, since no 

 chromosomal locus is found transferred after 

 the Hfr locus, we can conclude that Hfr is 

 always located at the terminus of a chromo- 

 some being transferred. 



Several Hfr strains derived from F^ cul- 

 tures show a very high frequency of recom- 

 bination. 4 In these strains, recombination 

 rates for the markers furthest from O occur 

 with a frequency of one to two per cent — 

 a rate at least one hundred times that found 



* See A. L. Taylor and E. A. Adelberg ( 1960). 



figure 24-3. Recombination percentages for 

 certain Hfr strains. = point of origin; — = 

 untested. (After A. L. Taylor and E. A. Adel- 

 berg, 1960. See References.) 



in other Hfr strains. Even so, less than 

 about one per cent of the progeny from mat- 

 ing these Hfr with F are Hfr. By artificial 

 rupture experiments, the sequence of certain 

 marker genes can be determined for each 

 of the three independently-arisen Hfr strains 

 of this type. 



These sequences are shown in Figure 

 24-3 together with the frequency of recom- 

 binants per one hundred Hfr cells in the 

 mating mixture. 



The results show that the markers held 

 in common by the three different Hfr strains 

 are in the same sequence. The O point, 

 however, is in a different position in each 

 case! Accordingly, so is the position of the 



