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The relationship of the sequences to one another is explained if each map is the segment of a circle. This was the first indication that bacterial chromosomes are circular. Furthermore, Allan Campbell proposed a startling hypothesis that accounted for the different Hfr maps. He proposed that if F is a ring, then insertion might be by the simple crossover between F and the bacterial chromosome (Figure 5-12). That being the case, any of the linear Hfr chromosomes could be generated simply by insertion of F into the ring in the appropriate place and orientation (Figure 5-13).

Several hypotheses—later supported—followed from Campbell’s proposal. 1. One end of the integrated F factor would be the origin, where the transfer of the Hfr chromosome begins. The terminus would be at the other end of F. 2. The orientation in which F is inserted would determine the order of entry of donor alleles. If the circle contains genes A, B, C, and D, then insertion between A and D would give the order ABCD or DCBA, depending on orientation. Check the different orientations of the insertions in Figure 5-12. How is it possible for F to integrate at different sites? 

If F DNA had a region homologous to any of several regions on the bacterial chromosome, any one of these could act as a pairing region at which pairing could be followed by a crossover. These regions of homology are now known to be mainly segments of transposable elements called insertion sequences. For a full explanation of these, see Chapter 13. The fertility factor thus exists in two states: 1. The plasmid state: as a free cytoplasmic element F is easily transferred to F recipients. 2. The integrated state: as a contiguous part of a circular chromosome F is transmitted only very late in conjugation. The E. coli conjugation cycle is summarized in Figure 5-14. 

Mapping of bacterial chromosomes BROAD-SCALE CHROMOSOME MAPPING USING TIME OF ENTRY Wollman and Jacob realized that it would be easy to construct linkage maps from the interrupted-mating results, using as a measure of “distance” the times at which the donor alleles first appear after mating. The units of map distance in this case are minutes. Thus, if b begins to enter the F cell 10 minutes after a begins to enter, then a and b are 10 units apart. Like eukaryotic maps based on crossovers, these linkage maps were originally purely genetic constructions. At the time they were originally devised, there was no way of testing their physical basis. 

FINE-SCALE CHROMOSOME MAPPING BY RECOMBINANT FREQUENCY For an exconjugant to acquire donor genes as a permanent feature of its genome, the donor fragment must recombine with the recipient chromosome. However, note that time-of-entry mapping is not based on recombinant frequency. Indeed the units are minutes, not RF. Nevertheless, it is possible to use recombinant frequency for a more fine-scale type of mapping in bacteria, and this is the method to which we now turn

First, we need to understand some special features of the recombination event in bacteria. Note that recombination does not take place between two whole genomes, as it does in eukaryotes. In contrast, it takes place between one complete genome, from the F, called the endogenote, and an incomplete one, derived from the Hfr donor and called the exogenote. The cell at this stage has two copies of one segment of DNA—one copy is the exogenote and one copy is part of the endogenote. 

Thus at this stage, the cell is a partial diploid, called a merozygote. Bacterial genetics is merozygote genetics. A single crossover in a merozygote would break the ring and thus not produce viable recombinants, as shown in Figure 5-15. To keep the circle intact, there must be an even number of crossovers. An even number of crossovers produces a circular, intact chromosome and a fragment

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