Learning Objective

Describe the mechanism of bacterial conjugation, differentiate F⁺ and Hfr donor cells, explain the roles of the F factor, tra genes, and oriT, and identify how DNA is transferred to an F⁻ recipient.

Conjugation is direct, cell-to-cell gene transfer between bacteria. A donor cell transfers a single strand of DNA to a recipient through physical contact.

Donor Cell Types

F⁺ Cells (Fertility Factor in a Plasmid)

• Carry the fertility plasmid known as the F factor.
• The F factor contains the tra region, which encodes:
  • Sex pilus for contact with the recipient.
  • Stabilizing proteins to maintain mating pairs.
  • DNA transfer machinery.
• DNA transfer begins at the origin of transfer (oriT) after a single-strand nick.

Hfr Cells (High-Frequency Recombination)

• It occurs when the F factor is integrated into the bacterial chromosome.
• The integrated F factor is referred to as an episome.
• Transfer begins at oriT, but because the F factor is embedded inside the chromosome, the donor transfers chromosomal genes first, not the F factor itself.

Activity

Recipient Cell

F⁻ Cell

• Lacks the F factor.
• Must be present in every conjugation pairing.
• Receives a single DNA strand from either F⁺ or Hfr donor.

Key Mechanisms

• F factor plasmids have insertion sequences, allowing them to integrate into the chromosome → forming Hfr cells.
• F⁺ × F⁻ → usually makes the recipient F⁺.
• Hfr × F⁻ → transfers chromosomal genes, not the fertility factor → recipient typically remains F⁻.

Activity