User:Spjelgus/draft
From Wikipedia, the free encyclopedia
MicroRNAs (miRNAs)are short ribonucleic acid (RNA) molecules, on average only 22 nucleotides long and are found in all eukaryotic cells. miRNAs are post-transcriptional regulators that bind to complementary sequences on target messenger RNA transcripts (mRNAs), usually resulting in translational repression and gene silencing.[1][2] The human genome may encode over 1000 miRNAs,[3][4] which may target about 60% of mammalian genes[5] and are abundant in many human cell types.[6]
![Thumb image](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a7/Microrna_secondary_structure.png/640px-Microrna_secondary_structure.png)
miRNAs show very different characteristics between plants and metazoans. In plants the miRNA complementarity to its mRNA target is nearly perfect, with no or few mismatched bases. In metazoans on the other hand miRNA complementarity is far from perfect and one miRNA can target many different sites on the same mRNA or on many different mRNAs. Another difference is the location of target sites on mRNAs. In metazoans the miRNA target sites are in the three prime untranslated regions (3'UTR) of the mRNA. In plants targets can be located in the 3' UTR but are more often in the coding region itself.[7] MiRNAs are well conserved in eukaryotic organism and are thought to be a vital and evolutionary ancient component of genetic regulation.[8][9][10][11]
The first miRNAs were characterized in the early 1990s, but miRNAs were not recognized as a distinct class of biologic regulators with conserved functions until the early 2000s. Since then, miRNA research has revealed multiple roles in negative regulation (transcript degradation and sequestering, translational suppression) and possible involvement in positive regulation (transcriptional and translational activation). By affecting gene regulation, miRNAs are likely to be involved in most biologic processes.[12][13][14][15][16][17][18]Different sets of expressed miRNAs are found in different cell types and tissues.[19]
Aberrant expression of miRNAs has been implicated in numerous disease states, and miRNA-based therapies are under investigation.[20][21][22]