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A micro RNA (abbreviated miRNA) is a small non-coding RNA molecule (containing about 22 nucleotides) found in plants, animals, and some viruses, which functions in RNA silencing and post-transcriptional regulation of gene expression. Encoded by eukaryotic nuclear DNA in plants and animals and by viral DNA in certain viruses whose genome is based on DNA, miRNAs function via base-pairing with complementary sequences within mRNA molecules. As a result, these mRNA molecules are silenced by one or more of the following processes: 1) cleavage of the mRNA strand into two pieces, 2) destabilization of the mRNA through shortening of its poly(A) tail, and 3) less efficient translation of the mRNA into proteins by ribosomes.〔 miRNAs resemble the small interfering RNAs (siRNAs) of the RNA interference (RNAi) pathway, except miRNAs derive from regions of RNA transcripts that fold back on themselves to form short hairpins, whereas siRNAs derive from longer regions of double-stranded RNA.〔 The human genome may encode over 1000 miRNAs,〔(''Homo sapiens'' miRNAs in the miRBase ) at Manchester University〕 which are abundant in many mammalian cell types and appear to target about 60% of the genes of humans and other mammals. miRNAs are well conserved in both plants and animals, and are thought to be a vital and evolutionarily ancient component of genetic regulation. While core components of the microRNA pathway are conserved between plants and animals, miRNA repertoires in the two kingdoms appear to have emerged independently with different primary modes of action. Plant miRNAs usually have near-perfect pairing with their mRNA targets, which induces gene repression through cleavage of the target transcripts. In contrast, animal miRNAs are able to recognize their target mRNAs by using as little as 6–8 nucleotides (the seed region) at the 5' end of the miRNA,〔 which is not enough pairing to induce cleavage of the target mRNAs.〔 Combinatorial regulation is a feature of miRNA regulation in animals.〔 A given miRNA may have hundreds of different mRNA targets, and a given target might be regulated by multiple miRNAs.〔 The first miRNA was discovered in the early 1990s. However, miRNAs were not recognized as a distinct class of biological regulators until the early 2000s. Since then, miRNA research has revealed different sets of miRNAs expressed in different cell types and tissues〔 and has revealed multiple roles for miRNAs in plant and animal development and in many other biological processes.〔 Aberrant expression of miRNAs has been implicated in numerous disease states, and miRNA-based therapies are under investigation. Estimates of the average number of unique messenger RNAs that are targets for repression by a typical microRNA vary, depending on the method used to make the estimate, but several approaches show that mammalian miRNAs can have many unique targets. For example, an analysis of the miRNAs highly conserved in vertebrate animals shows that each of these miRNAs has, on average, roughly 400 conserved targets.〔 Likewise, experiments show that a single miRNA can reduce the stability of hundreds of unique messenger RNAs, and other experiments show that a single miRNA may repress the production of hundreds of proteins, but that this repression often is relatively mild (less than 2-fold). ==History== The first miRNA was discovered in 1993 by Victor Ambros, Rosalind Lee and Rhonda Feinbaum during a study of the ''lin-4'' gene, which was known to control the timing of ''C. elegans'' larval development by repressing the ''lin-14'' gene.〔 When they isolated the ''lin-4'' gene, they found that instead of producing an mRNA encoding a protein, it produced short noncoding RNAs, one of which was a ~22-nucleotide RNA that contained sequences partially complementary to multiple sequences in the 3' UTR of the ''lin-14'' mRNA.〔 This complementarity was proposed to inhibit the translation of the ''lin-14'' mRNA into the LIN-14 protein. At the time, the ''lin-4'' small RNA was thought to be a nematode idiosyncrasy. Only in 2000 was a second small RNA characterized: ''let-7'' RNA, which represses ''lin-41'' to promote a later developmental transition in ''C. elegans''.〔 The ''let-7'' RNA was soon found to be conserved in many species, leading to the suggestion that ''let-7'' RNA and additional "small temporal RNAs" might regulate the timing of development in diverse animals, including humans.〔 A year later, the ''lin-4'' and ''let-7'' RNAs were found to be part of a very large class of small RNAs present in ''C. elegans'', ''Drosophila'' and human cells.〔〔〔 The many newly discovered RNAs of this class resembled the ''lin-4'' and ''let-7'' RNAs, except their expression patterns were usually inconsistent with a role in regulating the timing of development, which suggested that most might function in other types of regulatory pathways. At this point, researchers started using the term “microRNA” to refer to this class of small regulatory RNAs.〔〔〔 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「MicroRNA」の詳細全文を読む スポンサード リンク
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