During conjugation in bacteria, a high-frequency recombination (Hfr) strain transfers chromosomal genes to an F– strain. After mating, the recipient rarely becomes fully Hfr. Which characteristic of Hfr × F– mating explains this observation?

• A The entire F factor is transferred first, then chromosomal genes, but conjugation bridges are unstable and break before completion
• B The F– strain degrades foreign DNA by restriction endonucleases before F factor can integrate
• C Hfr cells cannot produce pili necessary for mating with F– cells
• D F factor is integrated into the chromosome; transfer begins within F factor sequence, chromosomal genes transfer first, but the conjugative bridge breaks before the complete F factor tail transfers ✓

Explanation

In Hfr strains, the F plasmid is integrated into the bacterial chromosome. During conjugation, DNA transfer begins within the integrated F sequence and proceeds along chromosomal DNA in a linear fashion. Because the F factor is split upon integration, the leading part of F enters first (providing mating stability), then chromosomal genes transfer in order, and the trailing portion of the F factor would only transfer after the entire chromosome — a process requiring ~100 minutes that is almost never completed because the conjugative bridge is physically fragile and breaks spontaneously. Therefore, the recipient receives chromosomal markers (allowing genetic mapping by interrupted mating) but almost never the complete F factor needed to become Hfr or F+.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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