Question

When an Hfr cell conjugates with an F- usually there is no conversion of the F- to Hfr. Occasionally however it can happen and now you can have two Hfr. How can this rare conversion happen?

Answer 1

In High frequency of recombination or Hfr Strains, the F factor is integrated in the host chromosome of the cell. The F- cells, which lack the F factor. Normally, Hfr strains do not convert F- cells to Hfr strain. This is because The F factor is integrated with the host chromosome and hence, is not present as a separate entity in the cytoplasm.

Conjugation cannot transfer the entire host chromosome into a F- recipient cell, although some transfer can occur. Thus, F- cell do not normally form F+ or HFr cells after conjugation. However, sometimes the F plasmid that was integrated in the Hfr host chromosome, will excise from the chromosome and move into the cytoplasm as a modified F factor. This modified F factor called F prime also carries a few chromosomal genes from the host. When such a donor cell mates with the F- recipient cell, the F prime factor will now be transferred to the recipient cell. As the F prime contains host chromosomal genes, it can integrate into the recipient cell, converting F- cell to an Hfr cell. This creates two Hfr strains. F prime can also remain as a cytoplasmic F factor in recipient cell.