21. Under which case of conjugation, recombination for chromosomal genes can be expected in E.coli?
(1) F⁺ X F^–             (2) F⁺ X HFr
(3) F^– X HFr           (4) F^– X F^–

In E. coli conjugation, recombination for chromosomal genes occurs primarily in HFr (high frequency recombination) donor strains crossed with F- recipients, as the integrated F plasmid allows transfer of chromosomal DNA segments that integrate via homologous recombination. This process contrasts with F plasmid-only transfers in other matings, enabling genetic mapping and analysis of bacterial chromosomes. The correct answer is option (3) F- x HFr, where the HFr acts as donor to the F- recipient.​​

Option Analysis

• (1) F+ x F-: F+ cells carry an autonomous F plasmid that transfers to F- recipients, converting them to F+ but rarely mobilizing chromosomal genes due to low integration frequency (about 1/10,000), resulting in minimal chromosomal recombination.​

• (2) F+ x HFr: Both are donors (F+ has episomal F, HFr has integrated F), so no stable recipient exists for effective DNA transfer and chromosomal recombination.​

• (3) F- x HFr: HFr donor initiates conjugation, nicking at the integrated F origin and transferring a linear chromosomal strand starting from the oriT; partial transfer followed by recombination in the F- recipient yields high-frequency chromosomal recombinants, though full F transfer is rare due to bridge breakage.​

• (4) F- x F-: Neither cell has the F factor, preventing pilus formation and any conjugation-mediated DNA transfer or recombination.​

Conjugation Mechanism

HFr strains form when the F plasmid integrates into the E. coli chromosome, enabling rolling-circle replication and transfer of chromosomal genes proximal to the integration site first. Recombinants arise from double crossovers integrating donor DNA into the recipient chromosome, with RecBCD and other proteins processing exonucleases. This high-efficiency process (unlike low-frequency events in F+ x F-) underpins bacterial genetics studies.