A plasmid carrying the bla gene (encoding TEM-type beta-lactamase) and a conjugation gene is transferred between two strains of E. coli. Which genetic transfer mechanism is operative, and which step requires direct cell-to-cell contact?

• A Transduction via bacteriophage; requires no cell contact
• B Transformation; DNA taken up from environment after cell lysis
• C Transposition of Tn3 transposon between chromosomes; requires no plasmid transfer
• D Conjugation via sex pilus (F-pilus) forming a mating junction; requires direct cell-to-cell contact ✓

Explanation

Conjugation is the mechanism of plasmid transfer between bacteria that requires direct cell-to-cell contact. The F+ (or R+) donor forms a sex pilus that contacts the recipient, retracts to bring cells close, and a mating junction (cytoplasmic bridge) forms through which one strand of the plasmid is transferred. Bla (TEM beta-lactamase) genes on conjugative R-plasmids are a major mechanism of horizontal ampicillin resistance transfer among Enterobacteriaceae. Transduction uses bacteriophage as vector (no cell contact needed). Transformation involves uptake of free DNA from the environment by competent cells. Transposons move within or between DNA elements within a single cell.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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