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Horizontal gene transfer is common among bacteria, even very distantly-related ones. For example, this process is thought to be a significant cause of increased drug resistance; when one bacterial cell acquires resistance, it can quickly transfer the resistance genes to many species. Also enteric bacteria appear to exchange genetic material with each other within the gut in which they live. Horizontal gene transfer can occur through the following three of the most common mechanisms:

• Transformation, in which cells take up naked DNA, i.e. DNA that is not contained in a cell. This process is relatively common in bacteria, but less common in higher eukaryotes. Transformation is often used to insert novel genes into bacteria for experiments, or for industrial or medical applications. See also molecular biology and biotechnology.
• Transduction in which a virus or bacteriophage inserts its genetic material into a cell. This process is used to insert novel genes into higher eukaryotes, where transformation is generally not practical
• Bacterial conjugation in which a living bacterial cell donates genetic material to another.

Analysis of DNA sequences suggests that horizontal gene transfer has also occurred within eukaryotes, from their chloroplast and mitochondrial genome to their nuclear genome. As stated in the endosymbiotic theory, chloroplasts and mitochondria probably originated as bacterial endosymbiontAn endosymbiont (also known as intracellular symbiont) is any organism that lives within cells of another organism, i. forming an endosymbiosis . For instance, some nitrogen fixing bacteria (e. in Rhizobium Sinorhizobium and Bradyrhizobium live in plants,s of a progenitor to the eukaryotic cell. There is also recent evidence that the adzuki bean beetleAdephaga Archostemata Myxophaga Polyphaga many subgroups see Subgroups of the order Coleoptera For alternate meanings see: Beetle (disambiguation Beetles (order Coleoptera are one of the main groups of insects. The order has more species in it than any ot has somehow acquired genetic material from its (non-beneficial) endosymbiont WolbachiaWolbachia is a type of bacteria that infects many insect and arthropod species, a variety of isopod species, at least one species of mite, and possibly a worm species. It is notable for the fact that it significantly alters the reproductive capabilities o.

Horizontal gene transfer is a potentially confounding factorIn statistics, a confounding factor is a factor which is the common cause of two things that may falsely appear to be in a causal relationship. It is the cause of a spurious relationship . For example, ice cream consumption and murder rates are highly cor in inferring phylogenetic treeA phylogenetic tree is a tree showing the evolutionary interrelationships among various species or other entities that are believed to have a common ancestor. A phylogenetic tree is a form of a cladogram. In a phylogenetic tree, each node with descendantss based on the sequenceThis is a page about mathematics. For other usages of "sequence", see: sequence (non-mathematical). In mathematics, a sequence is a list of objects (or events) which have been ordered in a sequential fashion; such that each member either comes before, or of one geneDNA and to a chromosome (right). Introns are regions often found in eukaryote genes which are removed in the splicing process: only the exons encode the protein. This diagram labels a region of only 40 or so bases as a gene. In reality many genes are much. For example, given two distantly related bacteria that have exchanged a gene, a phylogenetic treeA phylogenetic tree is a tree showing the evolutionary interrelationships among various species or other entities that are believed to have a common ancestor. A phylogenetic tree is a form of a cladogram. In a phylogenetic tree, each node with descendants including those species will show them to be closely related because that gene is the same, even though most other genes have substantially diverged. For this reason, it is often ideal to use other information to infer robust phylogenies, such as the presence or absence of genes, or, more commonly, to include as wide a range of genes for phylogenetic analysis as possible.

For example, the most common gene to be used for constructing phylogenetic relationships in prokaryoteProkaryotes are mostly unicellular organisms without a nucleus, in contrast to eukaryotes, organisms that have cell nuclei and may be variously unicellular or multicellular. The difference between prokaryote and eukaryote cell structure is the most imports is the 16s rRNA gene, since its sequences tend to be conserved among members with close phylogenetic distances, but variable enough that differences can be measured. However, in recent years it has also been argued that 16s rRNA genes can also be horizontally transferred. Although this may be infrequent, validity of 16s rRNA-constructed phylogenetic trees must be reevaluated.