Bacterial conjugation
Horizontal gene transfer (HGT) refers to the transfer of genes between organisms in a manner other than traditional reproduction. Also termed lateral gene transfer (LGT), it contrasts with vertical transfer, the transmission of genes from the parental generation to offspring via sexual or asexual reproduction. HGT has been shown to be an important factor in the evolution of many organisms. It has been suggested that lateral gene transfer to humans from bacteria may play a role in cancer. Richardson and Palmer (2007) state: Horizontal gene transfer (HGT) has played a major role in bacterial evolution and is fairly common in certain unicellular eukaryotes. There are three main forms of horizontal transmission used to spread genes between members of the same or different species: conjugation (bacteria-to-bacteria transfer), transduction (viral-mediated transfer), and transformation (free DNA transfer).  Horizontal gene transfer is defined to be the movement of genetic material between bacteria other than by descent in which information travels through the generations as the cell divides. 1 Transduction-the transfer of DNA by phage requires that the donor and recipient share cell surface receptors for phage binding and thus is usually limited to closely related bacteria; the length of DNA transferred is limited by the size of the phage head. 1 Conjugation-the transfer of DNA mediated by conjugal plasmids or conjugal transposons; requires cell to cell contact but can occur between distantly related bacteria or even bacteria and eukaryotic cells; can transfer long fragments of DNA. 1 Transduction-the transfer of DNA by phage requires that the donor and recipient share cell surface receptors for phage binding and thus is usually limited to closely related bacteria; the length of DNA transferred is limited by the size of the phage head. Virtually all organisms can disseminate genetic material via vertical gene transfer in which genetic information is transmitted from parents to offspring. In contrast, the process of horizontal gene transfer — or lateral gene transfer — is not common to all species. In this type of gene transfer, genetic material is transmitted between organisms that are not parent and offspring. Instead, the two organisms are usually unrelated, and are often of different species. The following animation depicts the process of transformation: Conjugation is a process by which one bacterium transfers genetic material to another bacterium through direct contact. During conjugation, one of the bacterial cells serves as the donor of the genetic material, and the other serves as the recipient. Finally, transduction is a process by which a virus transfers genetic material from one bacterium to another bacterium. This process depends on a specific type of virus called a bacteriophage, which is capable of infecting bacterial cells and using them as hosts to produce more viruses. Bacterial conjugation is often regarded as the bacterial equivalent of sexual reproduction or mating since it involves the exchange of genetic material. During conjugation the donor cell provides a conjugative or mobilizable genetic element that is most often a plasmid or transposon. The Ti and Ri plasmids can also be transferred between bacteria using a system (the tra, or transfer, operon) that is different and independent of the system used for inter-kingdom transfer (the vir, or virulence, operon). Conjugation is mediated by transposons and plasmids, two types of genetic material which are distinct from and independent of the bacterial genome. There is some evidence that viruses can also transmit genetic information via horizontal gene transfer. In this type of gene transfer, genetic material is transmitted between organisms that are not parent and offspring. Instead, the two organisms are usually unrelated, and are often of different species. 1 Transformation-the uptake of naked DNA is a common mode of horizontal gene transfer that can mediate the exchange of any part of a chromosome; this process is most common in bacteria that are naturally transformable; typically only short DNA fragments are exchanged. 1 Transduction-the transfer of DNA by phage requires that the donor and recipient share cell surface receptors for phage binding and thus is usually limited to closely related bacteria; the length of DNA transferred is limited by the size of the phage head. 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. The Ti and Ri plasmids can also be transferred between bacteria using a system (the tra, or transfer, operon) that is different and independent of the system used for inter-kingdom transfer (the vir, or virulence, operon).
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genetic information transmitted laterally between bacteria