During conjugation in Gram-negative bacteria, the transfer of chromosomal genes (Hfr × F- cross) is almost always incomplete. The main reason is:
- A The F factor is too large to be completely transferred through the conjugal pilus
- B Restriction enzymes in the F- cell degrade incoming Hfr DNA before transfer is complete
- C Transfer of the Hfr chromosome requires ~100 minutes at 37°C, but conjugal pairs separate spontaneously before the entire chromosome (including the integrated F factor) can be transferred ✓
- D The F- recipient lacks the OmpA receptor required for pilus retraction after the first 30 minutes
Correct answer: C. Transfer of the Hfr chromosome requires ~100 minutes at 37°C, but conjugal pairs separate spontaneously before the entire chromosome (including the integrated F factor) can be transferred
Explanation
In an Hfr cross, the integrated F factor initiates chromosomal DNA transfer at the origin of transfer (oriT); the chromosome unwinds and is transferred linearly at a rate of approximately 1 minute per 20 kb. Transfer of the complete E. coli chromosome (~4.6 Mb) would require ~100 minutes, but conjugal pairs break apart randomly within 1–2 minutes on average. This explains why distal genes are transferred with very low frequency — gradient mapping of transfer frequency is used in interrupted mating experiments to construct chromosomal maps.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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