Hfr cell
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A hfr cell (also called hfr strain) is a bacterium with a conjugative plasmid (often F) integrated into its genomic DNA. Hfr is the abbreviation for high frequency recombination, which was first characterized by Luca Cavalli-Sforza. Unlike a normal F+ cell, hfr strains will, upon conjugation with a F- cell, attempt to transfer their entire DNA through the mating bridges (called pili) to the F- cell. This occurs because the F factor has integrated itself via an insertion point in the bacterial chromosome. Due to the F factor's inherent nature to transfer itself over the pilus during bacteria conjugation, the rest of the bacterial genome is dragged along with it, thus making such cells very useful and interesting in terms of studying gene linkage and recombination.
A structure as fragile as a mating bridge will, however, likely break, and so the transfer is rarely complete. Thus, the F- cell uses only part of the genomic DNA of the Hfr cell for recombination. Though there is some debate on the issue, the pili themselves do not seem to be the structures through which the actual exchange of DNA takes place; rather, some proteins seem to open a channel between the bacteria.
It should also be noted that the bacterial genome's rate of transfer through the pilus is constant; this allows molecular biologists and geneticists to use Hfr strain of bacteria (often E. Coli) to study genetic linkage and map the chromosome. The procedure commonly used for this is called interrupted mating.
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