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In genetics, miRNA (micro-RNA) is a form of single-stranded RNA which is typically 20-25 nucleotides long, and is thought to regulate the expression of other genes. miRNAs are RNA genes which are transcribed from DNA, but are not translated into protein. The DNA sequence that codes for an miRNA gene is longer than the miRNA. This DNA sequence includes the miRNA sequence and an approximate reverse complement. When this DNA sequence is transcribed into a single-stranded RNA molecule, the miRNA sequence and its reverse-complement base pair to form a double stranded RNA segment; overall this RNA structure is called a hairpin structure. The Dicer[?] enzyme then cuts the 20-25 nucleotide double stranded region out of the hairpin structure, to release the mature miRNA.

The function of miRNA appears to be gene regulation. For that purpose, a miRNA is complementary to a part of one or more messenger RNAs (mRNAs). The annealing of the miRNA to the mRNA results in a double-stranded RNA, which is not transcribed into protein. In many cases, it is believed, this double-stranded RNA somehow triggers the degradation of the mRNA transcript through a process similar to RNA interference. In either case, the miRNA prevents the protein biosynthesis of a specific gene, namely the one to whose mRNA it is complementary.

This effect was first described for the worm C. elegans in 1993 by R. C. Lee of Harvard University. As of 2002, miRNAs have been confirmed in various plants and animals, including C. elegans, human and the plant Arabidopsis thaliana. Genes have been found in bacteria that are similar in the sense that they control mRNA abundance or translation by binding an mRNA by base pairing, however they are not generally considered to be miRNAs because the Dicer enzyme is not involved.

In plants, so-called interference RNA (also small interfering RNA (siRNA)) is used to prevent the transcription of viral RNA. While this iRNA is double-stranded, the mechanism seems to be closely related to that of miRNA, especially taking the hairpin structures into account. iRNAs are also used to regulate cellular genes, as miRNAs do.


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