Final answer:
An Hfr strain is a modified E. coli cell with a F plasmid integrated into its chromosome, resulting in high-frequency recombination during conjugation. This is significant in genetics and biotechnology for gene transfer and has been instrumental in producing human insulin.
Step-by-step explanation:
An Hfr strain is a genetically altered E. coli cell in which an F plasmid has integrated into the host cell's chromosome. This integration can happen at specific sites in the DNA where there's enough sequence similarity between the plasmid and chromosomal DNA to allow recombination. As a result, the recipient cell becomes an Hfr cell, which stands for High-frequency recombination. This type of cell is characterized by a 1000 times greater frequency of recombination compared to F+ cells, which carry the F plasmid but do not integrate it into their chromosome. The integration sets up the cell to transfer chromosomal genes at a high frequency during conjugation.
Furthermore, the F plasmid can be imprecisely excised, creating an F' plasmid that may carry chromosomal DNA with it. During conjugation, this DNA is introduced into the recipient cell and could be recombined into the bacterial chromosome. The use of recombinant DNA technology, such as that employed in creating Hfr strains, has significant biotechnological applications including the production of insulin in E. coli cells.