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Extrachromosomal DNA is any DNA that is found outside of the nucleus of a cell. It is also referred to as extranuclear DNA or cytoplasmic DNA. Most DNA in an individual genome is found in chromosomes but DNA found outside of the nucleus also serves important biological functions. In prokaryotes, nonviral extrachromosomal DNA is primarily found in plasmids whereas in eukaryotes extrachromosomal DNA is primarily found in organelles. Mitochondrial DNA is a main source of this extrachromosomal DNA in eukaryotes.〔 Extrachromosomal DNA is often used in research of replication because it is easy to identify and isolate.〔 Extrachromosomal DNA was found to be structurally different from nuclear DNA. Cytoplasmic DNA is less methylated than DNA found within the nucleus. It was also confirmed that the sequences of cytoplasmic DNA was different from nuclear DNA in the same organism, showing that cytoplasmic DNAs are not simply fragments of nuclear DNA. In addition to DNA found outside of the nucleus in cells, infection of viral genomes also provides an example of extrachromosomal DNA. ==Prokaryotes== Although prokaryotic organisms do not possess a membrane bound nucleus like the eukaryotes, they do contain a nucleoid region in which the main chromosome is found. Extrachromosomal DNA exists in prokaryotes outside of the nucleoid region as circular or linear plasmids. Bacterial plasmids are typically short sequences, consisting of 1 kilobase (kb) to a few hundred kb segments, and contain an origin of replication which allows the plasmid to replicate independently of the bacterial chromosome. The total number of a particular plasmid within a cell is referred to as the copy number and can range from as few as two copies per cell to as many as several hundred copies per cell. Circular bacterial plasmids are classified according to the special functions that the genes encoded on the plasmid provide. Fertility plasmids, or f plasmids,allow for conjugation to occur whereas resistance plasmids, or r plasmids, contain genes that convey resistance to a variety of different antibiotics such as ampicillin and tetracycline. There also exists virulence plasmids that contain the genetic elements necessary for bacteria to become pathogenic as well as degradative plasmids that harbor the genes that allow bacteria to degrade a variety of substances such as aromatic compounds and xenobiotics. Bacterial plasmids can also function in pigment production, nitrogen fixation and the resistance to heavy metals in those bacteria that possess them. Naturally occurring circular plasmids can be modified to contain multiple resistance genes and several unique restriction sites, making them valuable tools as cloning vectors in biotechnology applications.〔 Circular bacterial plasmids are also the basis for the production of DNA vaccines. Plasmid DNA vaccines are genetically engineered to contain a gene which encodes for an antigen or a protein produced by a pathogenic virus, bacterium or other parasite. Once delivered into the host, the products of the plasmid genes will then stimulate both the innate immune response and the adaptive immune response of the host. The plasmids are often coated with some type of adjuvant prior to delivery to enhance the immune response from the host. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus ''Borrelia'' (to which the pathogen responsible for Lyme disease belongs), several species of the gram positive soil bacteria of the genus ''Streptomyces'', and in the gram negative species ''Thiobacillus versutus'', a bacterium that oxidizes sulfur. The linear plasmids of prokarykotes are found either containing a hairpin loop or a covalently bonded protein attached to the telomeric ends of the DNA molecule. The adenine-thymine rich hairpin loops of the ''Borrelia'' bacteria range in size from 5 kilobase pairs (kb) to over 200 kb and contain the genes responsible for producing a group of major surface proteins, or antigens, on the bacteria that allow it to evade the immune response of its infected host. The linear plasmids which contain a protein that has been covalently attached to the 5’ end of the DNA strands are known as invertrons and can range in size from 9 kb to over 600 kb consisting of inverted terminal repeats.〔 The linear plasmids with a covalently attached protein may assist with bacterial conjugation and integration of the plasmids into the genome. These types of linear plasmids represent the largest class of extrachromosomal DNA as they are not only present in certain bacterial cells, but all linear extrachromosomal DNA molecules found in eukaryotic cells also take on this invertron structure with a protein attached to the 5’ end.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Extrachromosomal DNA」の詳細全文を読む スポンサード リンク
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