Final answer:
An F+ cell becomes an Hfr cell by integrating its F plasmid into the host chromosome. This allows it to transfer chromosomal DNA to an F-cell during conjugation, which can lead to genetic recombination and is also used for mapping bacterial genomes.
Step-by-step explanation:
An F+ cell becomes an Hfr (High-frequency recombination) cell when the F plasmid integrates into the bacterial chromosome. This integration occurs at specific sites within the bacterial DNA, where there's sequence similarity between the F plasmid and the chromosomal DNA, allowing for recombination. Once the F plasmid is integrated, the cell can produce sex pili and form a conjugation tube with an F-cell, initiating a process where a single strand of the Hfr cell's chromosomal DNA, beginning at the original insertion site, is transferred to the recipient F- cell during conjugation.
The transferred chromosomal DNA has a higher likelihood of recombination within the recipient cell if it's closer to the insertion site, due to the temporary nature of the contact between the two cells. This process not only leads to genetic variation but also aids in mapping bacterial genomes, as the time it takes for genes to be transferred through conjugation is indicative of their relative positions on the chromosome.