Many probably did not know that bacteria can engage in sexual reproduction. This is not quite what you can imagine. Today we consider the process of bacterial conjugation, what is conjugation in microbiology and its effect on the genetic variation of bacteria.
Sexual reproduction of bacteria
This will not be a big shock, but bacteria do not have sexual reproduction, at least in the usual sense. Bacterial cells multiply by creating clones of themselves. A mother cell copies its DNA chromosome, then splits its cell in half, holding one chromosome and giving it to a new daughter cell. By convention, these cells are called mother and daughter cells, but in reality they are clones.
They have the same genetic material. In the bacterial population, this process continues, one cell divides into two again and again and again, resulting in huge populations that are all clones of each other. This is called vertical gene transfer when DNA is passed from mother to offspring. And this is what happens in nature the vast majority of the time.
Bacteria are sly little creatures
What is conjugation? Bacteria have some ingenious ways of generating genetic diversity. For example, bacteria are able to exchange genes with their neighbors.
This is called horizontal gene transfer and refers to the ability of some bacterial cells to acquire new genes from neighboring ones in their environment. Now, instead of being a daughter cell of the clone, the cell has a new genetic diversity, a mixture between the vertically transmitted DNA of the mother cells and the horizontally transferred neighboring cell DNA.
Remember that bacteria do not have sexual reproduction? In an evolutionary sense, this is crucial for allowing mixing and matching of genes, which leads to genetic diversity within the same species. Now we can see that the bacteria do not have the usual sex, they have horizontal gene transfer mechanisms to generate genetic diversity. In biology, this is conjugation.
Horizontal gene transfer
There are three ways that bacteria can perform horizontal gene transfer:
- transduction uses bacterial viruses called bacteriophages to transfer DNA from one infected cell to another;
- transformation is the ability of some cells to accept free-floating DNA found in the environment;
- conjugation allows you to transfer DNA through a structure called a pylus, that is, from one cell to another.
What is conjugation?
For conjugation, two living bacterial cells must come into direct contact with each other. Contact between cells is carried out by pairing a special appendage (pilus), resembling hair on the surface of a bacterium. It has a protein base, similar to a hair structure that extends from a bacterial cell.
Some appendages are used to attach to surfaces, but special conjugation is used exclusively to attach to other cells and facilitate DNA transfer.
The cell that will transmit the DNA is called a donor cell and builds a conjugated pilus. Its conjugation is a hollow tubular structure that connects the cytoplasm of the donor cell with the cytoplasm of the recipient.
Plasmid transfer
When the cytoplasm of the donor cell and the recipient is physically connected, it is time for DNA transfer. If the donor cell contains a plasmid, a round part of extrachromosomal DNA, then it can be transferred to the recipient cell. This is done by copying the plasmid and sending the chain of the copied DNA to the recipient via pairing of the pilus. The end result is a copy of the plasmid in both the donor and the recipient. Perhaps most interesting is the fact that the plasmid carries genes that allow the recipient cell to become the conjugation donor itself! Now, the recipient cell can also spread the plasmid to new cells that it encounters in its environment.
The value of conjugation
Bacterial conjugation is the transfer of genetic material between bacterial cells through direct contact or a bridge-like connection. This is the mechanism of horizontal gene transfer, as is transformation and transduction. Bacterial conjugation is often regarded as the bacterial equivalent of sexual reproduction or mating, as it is associated with the exchange of genetic material. However, this is not sexual reproduction, as the gamete exchange does not occur.
During conjugation (microbiology), a donor cell provides a conjugative or mobilized genetic element, which is most often a plasmid or transposon. Most conjugative plasmids have systems that ensure that the recipient cell no longer contains a similar element. Transmitted genetic information is often beneficial to the recipient. Benefits may include antibiotic resistance, xenobiotic tolerance, or the ability to use new metabolites.
Such useful plasmids can be considered as bacterial endosymbionts. And other elements like bacterial parasites, conjugation as a mechanism developed by them to ensure their spread. This process was discovered in 1946 by Joshua Lederberg and Edward Tatum.
Conjugation mechanism
First, the donor cell produces a pilus. It joins the receiving cell and combines the two cells. The mobile plasmid is serrated and one strand of DNA is then transferred to the recipient cell. Both synthesize a complementary chain to produce a circular plasmid, and also propagate piluses. Both cells are currently a viable donor for the F-factor.
An F-plasmid is an episome (a plasmid that can integrate into a bacterial chromosome by homologous recombination). It carries the origin of replication and the source of transmission. In a given bacterium, free or integrated, there can be only one copy of the F plasmid, and bacteria with the copy are called F-positive and are designated F + . Cells that lack F-plasmids are called F-negative (F - ); they can function as recipient cells.
Although there is some debate about the exact conjugation mechanism, it may seem that piluses are not the structures through which DNA is exchanged, but nevertheless, DNA transformation continues. A few locally encoded proteins open a channel between bacteria. It is believed that the enzyme located at the base of the pilus initiates membrane fusion.
Examples
A striking example of conjugation is demonstrated by bacteria associated with nitrogen-fixing trizobia, which are an interesting case of internal conjugation. For example, the tumor inducing (Ti) Agrobacterium plasmid and the root tumor inducing (Ri) A. rhizogenes plasmid contain genes that are capable of transferring plant cells. The expression of these genes effectively converts plant cells into opin-producing plants. Opins are used by bacteria as sources of nitrogen and energy. Infected cells form coronary bile or root tumors.
Thus, the plasmids Ti and Ri are endosymbionts of bacteria, which in turn are parasites of the infected plant. Plasmids Ti and Ri can also be transferred between bacteria. Such transfers create virulent strains from previously avirulent strains.
What is conjugation? This is a convenient means for transferring genetic material with many purposes. Successful transmission from bacteria to yeast, plants, mammalian cells, diatoms and isolated mammalian mitochondria has been reported. Conjugation has advantages over other forms of genetic transfer, including minimal disruption of the target cell wall and the ability to transfer relatively large amounts of genetic material.