Conjugation is the genetic transfer mechanism by which one bacterium transfers genetic material to another bacterium through direct cell to cell contact. Other types of genetic transfer mechanisms include transformation and transduction. During the process of conjugation, one bacterium serves as the donor of the genetic material of interest, and the other bacterium serves as the recipient organism. The donor bacterium carries a DNA sequence called the fertility factor, or F-factor. Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. This takes place through a pilus (plural: pili). Bacterial conjugation is a sexual mode of genetic transfer in the sense that chromosomal material from two sexually distinct types of cells are brought together in a defined and programmed process. In conjugation, DNA is transferred between bacteria through a tube between cells. This tube formed between both cells (i.e., the donor and recipient bacterial cell) is known as sex pilus or just pilus.
MECHANISM OF CONJUGATION
Conjugation is the process by which one bacterium transfers genetic material to another through direct contact. During conjugation, one bacterium serves as the donor of the genetic material, and the other serves as the recipient. The donor bacterium carries a DNA sequence called the fertility factor, or F-factor. The F-factor allows the donor to produce a thin, tube-like structure called a pilus, which the donor uses to contact the recipient. The pilus then draws the two bacteria together, at which time the donor bacterium transfers its genetic material (DNA) to the recipient bacterium. Typically, the genetic material is in the form of a plasmid, or a small, circular piece of DNA. The genetic material transferred during conjugation often provides the recipient bacterium with some sort of genetic advantage. For instance, in many cases, conjugation serves to transfer plasmids that carry antibiotic resistance genes to the recipient organism. In conjugation, DNA is transferred from one bacterium to another (Figure 1). After the donor cell pulls itself close to the recipient using a structure called a pilus, DNA is transferred between cells. In most cases, this DNA is in the form of a plasmid.
Figure 1. Illustration of Conjugation
PROCESS OF CONJUGATION
- An F+ donor cell contains its chromosomal DNA and an F plasmid. It has a rod-like pilus. A recipient F- cell has only a chromosome and no F plasmid.
- The donor cell uses its pilus to attach to the recipient cell, and the two cells are pulled together.
- A channel forms between the cytoplasm of the two cells, and a single strand of the F plasmid is fed through.
- Both of the cells now have an F plasmid and are F+. The former recipient cell is now a new donor and can form a pilus.
Donor cells typically act as donors because they have a chunk of DNA called the fertility factor (or F factor). This chunk of DNA codes for the proteins that make up the sex pilus (Figure 1). If the F factor is transferred during conjugation, the receiving cell turns into an F+ cell.
Application of conjugation
Conjugation is a convenient means for transferring genetic material to a variety of targets. In laboratories, successful transfers have been reported from bacteria to yeast, plants, mammalian cells, diatoms and isolated mammalian mitochondria. Conjugation has advantages over other forms of genetic transfer including minimal disruption of the target’s cellular envelope and the ability to transfer relatively large amounts of genetic material. In plant engineering, Agrobacterium-like conjugation complements other standard vehicles such as tobacco mosaic virus (TMV). While TMV is capable of infecting many plant families these are primarily herbaceous dicots. Agrobacterium-like conjugation is also primarily used for dicots, but monocot recipients are not uncommon.