Rear of F-plasmid
Front of F-plasmid
r oriT enters recipient first
Rear of F-plasmid guished phenotypically, usually by their growth properties. For example, the recipient may have a mutation that makes the bacteria unable to grow with only lactose as a carbon source. The donor Hfr strain would have the gene that restores the ability to grow on lactose. Using this method, genetic maps may be constructed by two major approaches. First, the cotransfer frequency of two genes may be measured. For example, if gene a and b are close to each other, the donor Hfr strain would transfer these two genes together at a high frequency. Alternately, if gene a and b were on opposite sides of the chromosome, the donor Hfr strain would usually only transfer gene a, and the cotransfer frequency would be low.
Secondly, time of entry measurements may be made. Genes are transferred starting at the site where the F-plasmid is integrated and proceeding sequentially around the circular chromosome (Fig. 18.17). The length of time it takes for a series of genes to enter the recipient cell gives an estimate of their relative distance from the origin of transfer of the Hfr strain used. In order to determine the time of entry by conjugation the site and orientation of the F-plasmid must be known. In addition, mutations in the genes being studied (a, b, c, and d) must give recognizable phenotypes, such as the ability to grow using lactose as a carbon source or a requirement for some nutrient. Finally, the recipient must be resistant to some antibiotic (e.g., streptomycin) so that it can be selected on medium that prevents growth of the Hfr strain. Different Hfr strains will transfer the same genes in different orders and at different times, depending on their location relative to the site of integration of the F plasmid.
cotransfer frequency Frequency with which two genes remain associated during transfer of DNA between cells
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