A plasmid is a small, circular piece of extra chromosomal DNA that self-replicating and contains a limited number of genes.
Plasmids that control such characteristics as fertility are called F factors, and those that contain genes for transferable drug resistance are known as R factors.
To date no Gram-positive cells have been shown to conjugate; however, many F factor containing Gram-negative bacteria may undergo plasmid-controlled conjugation.
Microbes which contain the F factor are designated F+, or males, since they serve as donors. Those that lack the F factor are designated as F, or females, since they serve as recipients. When the F factor, is donated to an F– cell, the female becomes a male, or F+.
The first stage in plasmid-controlled conjugation involves the attachment of the two cells. The sex pilus, a filamentous structure extending from the cell wall of the F+ cell, attaches to the cell wall of an F bacterium.
A conjugation tube is formed between the cells and as this tube is formed, the plasmid is replicated inside the donor cell. This process takes place in the same way that the host nucleoid is duplicated.
One of the F factor loops remains attached to the inner surface of the F+ cell while the other plasmid is free to move through the conjugation tube into the recipient cell. After the transfer has been completed and the cells separate, both the recipient and the donor contain plasmids.
The percentage of F factor-containing bacteria in a population increases if the microbes are crowded into close contact. Plasmid-controlled conjugation occurs more easily and successfully within the Gram-negative enteric species found as normal flora in the intestinal tract.
These bacteria show a great amount of genetic variety. Bacterial populations that lack this close contact usually have a lower rate of conjugation, fewer F factors, and less genetic variety.
In some cases, the F factor plasmid may become integrated into the endogenote DNA. These cells have been carefully studied, and the frequency with which they genetically recombine with F– cells is 1000 times greater in comparison of F+ cells.
Therefore, these cells are called Hfr bacteria, or high-frequency recombinants. During conjugation between an Hfr and an F- cell, all of the genetic material in the Hfr undergoes DNA duplication. One of the loops is then broken within the Hfr gene sequence and being to move through the conjugation tube into the F” recipient.
The entire length of DNA rarely moves into the recipient, since the two cells are being held together by such a fragile connection. Therefore, it is highly unlikely that the recipient will receive the entire Hfr sequence and be converted to an Hfr or F cell.
More importantly, the P cell will receive new genes from the Hfr cell that can be genetically recombined with the endogenote. When this recombination occurs, the recipient will have new genes and gene combinations not previously found in the population.
After reproduction by binary fission, a complete line of offspring will be produced that have these new characteristics. Any population of microbes produced by asexual reproduction from a single parent cell is called a clone.
Genetic variation may also be produced in conjugating populations if the Hfr cell converts to another form containing a plasmid known as F’ (“F prime”).
An F’ plasmid is formed when the Hfr segment detaches from the host DNA loop and mistakenly carries with it some of the adjacent host genes. When this happens, the newly formed F plasmid contains endogenote genes from the host cell that may be transferred to other F– cells during regular conjugation.
These genes may then be recombined with the genes of the recipient cell. Plasmid- controlled conjugation generates genetic variation in a number of ways. The most important outcome of this form of gene transfer is the formation of new gene combinations.
If these combinations provide the cell with a selective advantage, such as resistance of drugs, the recombinant microbe may produce a clone that could have a great medical significance.