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Copy-number variations (CNVs)—a form of structural variation—are alterations of the DNA of a genome that results in the cell having an abnormal or, for certain genes, a normal variation in the number of copies of one or more sections of the DNA. CNVs correspond to relatively large regions of the genome that have been deleted (fewer than the normal number) or duplicated (more than the normal number) on certain chromosomes. For example, the chromosome that normally has sections in order as A-B-C-D might instead have sections A-B-C-C-D (a duplication of "C") or A-B-D (a deletion of "C").This variation accounts for roughly 13% of human genomic DNA and each variation may range from about one kilobase (1,000 nucleotide bases) to several megabases in size. CNVs contrast with single-nucleotide polymorphisms (SNPs), which affect only one single nucleotide base. ==Sources== Most CNVs are stable and heritable, so CNV between individuals is largely a product of genetic heritage, however,'' de novo'' CNVs arise through diverse mechanisms at various stages of development. Multiple homologous recombination reactions on each chromosome are required for the meiotic cell divisions that give rise to gametes, and although these events are of very high fidelity, occasional mistakes are inevitable. Therefore, most CNV in the human genome likely arises through non-allelic homologous recombination events in which unmatched regions of chromosomes are mistakenly recombined during meiosis. However, two lines of evidence suggest that this is not the whole story. Firstly, various studies have revealed extensive CNV between different cells in the same individuals; these CNVs must have arisen post-fertilisation. Secondly, some complex genetic rearrangements cannot be readily reconciled with a non-allelic homologous recombination mechanism; these have been proposed to arise through rare replication defects resulting from broken DNA at one replication fork invading another fork, resulting in a template switch.〔 reported in (【引用サイトリンク】 publisher = ScienceDaily )〕 This was subsequently superseded by a more general microhomology-mediated break-induced replication (MMBIR) model. CNVs can be caused by structural rearrangements of the genome such as deletions, duplications, inversions, and translocations. Low copy repeats (LCRs), which are region-specific repeat sequences, are susceptible to such genomic rearrangements resulting in CNVs. Factors such as size, orientation, percentage similarity and the distance between the copies influence the susceptibility of LCRs to genomic rearrangement. Segmental Duplications (SDs) map near ancestral duplication sites in a phenomenon called duplication shadowing which describes the observation of a ~10 fold increased probability of duplication in regions flanking duplications versus other random regions. CNV in short repeated DNA sequences called microsatellites can arise through additional mechanisms including replication slippage and defective mismatch repair. The resulting microsatellite instability is characteristic of some cancers and underlies a family of genetic disorders including Huntington's disease and myotonic dystrophy. 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「Copy-number variation」の詳細全文を読む スポンサード リンク
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