Conjugation is one the three mechanism of DNA exchange between bacteria, the other being transformation and transduction.
Conjugation occurs between two living cells, involves cell to cell contact, and requires mobilization of either a plasmid or a chromosome of donor bacterial cells.
Conjugative plasmid transfer themselves between bacteria, which had lead to spread of antibiotic resistance among pathogenic bacteria.
Before talking about the mechanism of conjugation lets let’s first remind some of the key information related to plasmid.
- Plasmids are small, circular pieces of DNA that are separate and replicate independently from the bacterial chromosome.
- Plasmid carry genes associated with specialized functions such as drug resistance.
- Plasmid may encode genes that mediate their transfer from one organism to another but not all plasmid are capable of conjugative transfer
- Plasmids which can transmit from one cell to another cell independently are called self transmissible plasmids. Non-transmissible plasmids are not able to go independent transmission.
The F plasmid also called ‘fertility factor’ confers donor characteristics (sex pilus) to bacterial cells.
- Bacteria that have F plasmid are referred to as as F+ or male.
- Those that do not have F plasmid are called F- or female.
Mechanism of plasmid mobilization by conjugation
- Conjugative plasmids initiate gene transfer by altering the cell surface to allow contact between the plasmid-containing donor cell (F+ or male) and a plasmid lacking recipients (F- or female)
- Sex pilus originates from the donor and establishes conjugative bridge (temporary cytoplasmic bridge) that serves as the conduit for DNA transfer from donor to recipient bacterial cell
- Intercellular contact established
- A copy of DNA from donor cell (F+ Cell) is transferred to recipient cell (F- cell).
- Complementary DNA strand is synthesized in both donor cell and recipient cell.
- As the recipient cell now contains F plasmid it behaves as a donor cell.
If F- cells and F+ cells are mixed in a culture, the entire population quickly becomes F +