Features of interrupted mating between Hfr x F– cells of E. coli

The important features of interrupted mating between Hfr x F– cells of E. coli are briefly mentioned below:

(a) Transfer of Hfr chromosome to P cell begins at a particular point on chromosome determined by the site of integration of the F-plasmid. This means that in different Hfr-strains, transfer begins at different joints of Hfr-chromosome.

(b) The integrated F-DNA nicks at the OriT locus and a part of the F-DNA forms the leading end of the Hfr-chromosome being transferred (see Fig. 9.107). The chromosomal genes enter into the recipient in a linear order. The first gene is the one just behind the part of F-DNA forming the leading end. Other genes follow successively.

(c) The time of entry of any specific gene is the time taken by it to enter the recipient calculated from the time of mixing the Hfr and F– cells. The time of entry, expressed in minutes, is determined by interrupted mating technique.

(d) The number of F– cells showing the presence of a specific gene transferred from Hfr-donor increases with time till a maximum is reached. This happens because all donor cells of an Hfr-strain do not start transferring genes at the same time. The maximum is reached because all do not cells present in the population have transferred the particular gene to the recipients. A graphical representation of entry of two hypothetical genes x and y is shown in Fig. 9.109.

(e) Though under appropriate selective conditions, a gene entering into a recipient cell can be expressed as soon as it enters, permanent genetic recombination occurs only after the gene has been incorporated into the F–-chromosome by homologous recombination.

The incoming genes, transferred from Hfr strain, replace homologous genes of F–. The replaced segments of DNA containing these genes, as well as the segments of Hfr-chromosome which have not been incorporated into the F– chromosome, are eventually hydrolysed by cellular nucleases.

(f) In different Hfr-strains of E. coli, the F-plasmid is integrated at different loci of the chromosome. There are numerous sites where the F can be integrated. As a result, the linear order of genes during entry into recipient is different in each strain of Hfr. Moreover, the F-plasmid can be integrated in two different orientations. This also changes the linear order of genes.