24. Which one of the following is correct statement about the difference between a F+ cell and Hfr cell?
(1) In F+ cell F factor is an integral part of bacterial chromosome, while in Hfr cell F factor is present as an episome.
(2) F+ and Hfr are synonymous.
(3) In F+ cell a bacteriophage carries F factor, while in Hfr cell F factor is an integral part of bacterial chromosome.
(4) In F+ cell F factor is in the form of an episome while in Hfr cell, F factor is integrated into the bacterial chromosome.
The correct answer is (4) In F+ cell F factor is in the form of an episome while in Hfr cell, F factor is integrated into the bacterial chromosome.
Option Analysis
Option (1): Incorrect, as it reverses the states—F factor integrates into the chromosome in Hfr cells, not F+ cells, where it remains extrachromosomal.
Option (2): Wrong, since F+ and Hfr cells differ fundamentally in F factor location and conjugation behavior; F+ transfers only the plasmid, while Hfr transfers chromosomal DNA.
Option (3): False, as neither involves a bacteriophage; F factor transfer occurs via conjugation pilus, not phage-mediated transduction.
Option (4): Correct, because F+ cells carry the F plasmid (episome) independently, enabling plasmid-only transfer, whereas Hfr cells form when the episome integrates into the chromosome via recombination, leading to high-frequency chromosomal gene transfer.
F+ and Hfr Cell Basics
F+ cells contain the F plasmid as a free episome, allowing them to act as donors in conjugation by transferring the plasmid to F- recipients, converting them to F+. Hfr (high frequency of recombination) cells arise when this episome integrates into the bacterial chromosome through insertion sequences, so during conjugation, they transfer chromosomal genes starting from the integration site. This integration explains Hfr’s name, as partial chromosome transfer promotes recombination in recipients at higher rates than F+ conjugation.
Conjugation Implications
In F+ conjugation, the process completes quickly with full F factor transfer, but Hfr conjugation often interrupts, transferring only part of the chromosome without converting recipients to donors. These differences highlight bacterial horizontal gene transfer mechanisms, crucial for genetics exams like CSIR NET.
