Final answer:
Hfr cells promote high recombination levels in F minus bacterial populations by transferring chromosomal DNA near the integration site of the F plasmid during the transient process of conjugation, which often leads to the recombination of this DNA with the recipient's chromosome and can transform the recipient into an Hfr cell.
Step-by-step explanation:
Hfr cells promote a high level of recombination when added to a F minus (F−) population of bacteria due to their unique ability to integrate the F plasmid into their bacterial chromosome. When conjugation occurs with F− cells, Hfr cells begin transferring their chromosome at the integration site of the F plasmid. However, because bacterial conjugation is often brief and the entire chromosome is large and takes time to transfer, only parts of the chromosome near the integration site are typically transferred. This transferred DNA, including some of the host's chromosomal DNA, may then recombine with the recipient F− cell's chromosome.The integration of the F plasmid into the chromosomal DNA leads to a displacement of the host DNA by a process similar to rolling circle replication.
Since the F plasmid is integrated, genes closest to the integration point are more susceptible to being transferred and therefore recombine at a higher frequency in the recipient cells. This recombination process efficiently introduces new genetic material into the F− cell, which can include beneficial genes and can even result in the recipient cell itself becoming an Hfr cell, capable of further gene transfer through conjugation.Historically, this method was instrumental in mapping the E. coli genome, as the time individual genes took to transfer could be used to determine their relative positions on the bacterial chromosome.