Several hypotheses—later supported—followed from Campbell’s proposal. 1. One end of the integrated F factor would be the origin, where 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:
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.