Q.23 Which one of the following is correct in the case of conjugation of a high frequency
recombination (Hfr) strain with F– strain of E. coli?
(A) Recombination frequency is high, F factor transfer frequency is low
(B) Recombination frequency is high, F factor transfer frequency is high
(C) Recombination frequency is low, F factor transfer frequency is high
(D) Recombination frequency is low, F factor transfer frequency is low
The correct answer is (A) Recombination frequency is high, F factor transfer frequency is low.
In Hfr conjugation with F- E. coli, chromosomal genes transfer at high frequency due to F plasmid integration, leading to frequent recombination, but complete F factor transfer is rare as mating bridges break before the entire chromosome transfers.
Option Analysis
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(A) Recombination frequency is high, F factor transfer frequency is low: Hfr strains integrate the F plasmid into the chromosome, transferring chromosomal DNA first via rolling-circle replication; partial transfer allows high recombination in F- recipients via homologous recombination, but full F factor (at the trailing end) rarely completes due to conjugation interruption.
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(B) Recombination frequency is high, F factor transfer frequency is high: Incorrect, as complete F transfer requires the entire bacterial chromosome to transfer first, which seldom occurs before cell separation, keeping F transfer low.
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(C) Recombination frequency is low, F factor transfer frequency is high: Wrong, since Hfr enables high chromosomal recombination (1000x more than F+ x F-), not low; F transfer remains low.
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(D) Recombination frequency is low, F factor transfer frequency is low: Inaccurate for Hfr, which defines “high frequency recombination” through efficient chromosomal gene transfer and integration.
In Hfr strain conjugation with F- E. coli, bacterial genetics showcases high recombination frequency for chromosomal genes while F factor transfer remains low, a key concept for CSIR NET Life Sciences aspirants studying Hfr strain conjugation recombination frequency.
Hfr Conjugation Mechanism
Hfr (high frequency recombination) forms when the F plasmid integrates into the E. coli chromosome at oriT, initiating transfer of proximal chromosomal genes first during conjugation with F- recipients. The donor Hfr cell forms a sex pilus, nicks DNA, and rolls out single-stranded chromosomal DNA; recombination occurs via double crossovers in the F- cell, yielding stable recombinants, but the trailing F factor rarely transfers fully due to mating pair instability.
Why Recombination is High
Chromosomal segments transfer efficiently (up to 10^5-10^6 times more than F+ x F-), integrating via RecA-mediated homologous recombination; genes near the origin enter first, enabling mapping via interrupted mating. This high Hfr strain conjugation recombination frequency underpins bacterial gene order studies.
Why F Factor Transfer is Low
Complete transfer demands the full ~4.6 Mb chromosome plus F plasmid (last to enter), but bridges break after 20-100 minutes, transferring only partial DNA; recipients stay F-.
Exam Relevance for CSIR NET
This distinguishes Hfr from F+ (high F transfer, low recombination) and aids questions on bacterial genetics, gene mapping. Focus on Hfr strain conjugation with F- E. coli for scoring in molecular biology sections.
