Bacterial conjugation mediated by the F plasmid transfers genetic material. In an Hfr (high frequency recombination) × F− cross, what is the expected outcome regarding fertility factor transfer?

• A Chromosomal genes near the origin of transfer (oriT) are transferred at high frequency, but the complete F factor rarely transfers because conjugation terminates before the entire chromosome is transferred; recipient remains F− ✓
• B Complete F plasmid is transferred efficiently, converting all F− cells to F+
• C Only the antibiotic resistance genes flanking the F plasmid are transferred
• D No chromosomal DNA transfers; only F plasmid DNA transfers

Explanation

In Hfr strains, the F plasmid is integrated into the bacterial chromosome. During conjugation with F− cells, transfer begins at oriT on the integrated F factor and proceeds linearly through chromosomal DNA. Chromosomal genes proximal to oriT transfer first and at highest frequency. Because the entire chromosome (4.6 Mbp) takes ~100 minutes to transfer but conjugation typically breaks before completion, the distal end of the F factor (needed to convert recipient to F+) almost never transfers. Thus, Hfr × F− crosses produce F− transconjugants with new chromosomal markers but NOT F+ cells.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.