Conjugation: Transfer of Chromosomal DNA by High Frequency Recombination (HFr) Strain

The F+ strain which contains plasmid gene as an episome (i.e.  F plasmid becomes integrated into the host cell genome at one of the several possible sites chromosomal gene) induce more than thousand times the number of genetic recombination than seen in F+ and F- cells.  Such a donor strain is called a high frequency of recombination (HFr) strain.

1. The HFr strain is able to transfer some of the host genes to the recipient.
2. The sex pilus contacts the recipient F- cell and pulls the cells together
3. The donor chromosome is transferred as SS DNA starting at the origion of transfer. Gene that is closest to the origin is transferred first.
4. Segments of the integrated plasmid are at the beginning and the end of the DNA being transferred (Note: Yellow part is integrated plasmid.
5. To transfer the entire plasmid, entire bacterial gene must be transferred first. But the donor and recipient cells separate  prior to the complete transfer of genome  and plasmid
6. Usually only part of the F plasmid called the initiating segment is transferred along with adjacent chromosomal genes.
7. Transferred DNA becomes double stranded
8. The donor DNA is integrated into the recipient cell’s DNA by homologous recombination.
9. The recipient now carries transferred genes, but remain F- where as donor cell remains HFr.
10. The recipient cell does not become an F+ donor cell, as only a part of the F plasmid is transferred but the donor cells remain as Hfr strains.