The function of miRNAs appears to be in gene regulation. For that purpose, a miRNA is complementary to a part of one or more messenger RNAs (mRNAs). Animal miRNAs are usually complementary to a site in the 3' UTR whereas plant miRNAs are usually complementary to coding regions of mRNAs. Perfect or near perfect base pairing with the target RNA promotes cleavage of the RNA. This is the primary mode of plant microRNAs.
In animals, microRNAs more often only partially base pair and inhibit protein translation of the target mRNA (this exists in plants as well but is less common). For partially complementary microRNA to recognise their targets, the nucleotides 2–7 of the miRNA ('seed region'), still have to be perfectly complementary.
miRNAs occasionally also causes DNA methylation of promoter sites and therefore affecting the expression of targeted genes. miRNAs function in association with a complement of proteins collectively termed the miRNP. Human miRNPs contain eIF2C2 (also known as Argonaute 2), DDX20, GEMIN4 and microRNA.
Animal microRNAs target in particular developmental genes. In contrast, genes involved in functions common to all cells, such as gene expression, have very few microRNA target sites, and seem to be under selection to avoid targeting by microRNAs.
This effect was first described for the worm ''C. elegans'' in 1993 by Victor Ambros and coworkers. As of 2002, miRNAs have been confirmed in various plants and animals, including ''C. elegans'', human and the plant ''Arabidopsis thaliana''. Work at the University of Louisville has resulted in the production of microarrays dubbed MMChips containing all then known miRNAs for human, mouse, rat, dog, ''C. elegans'' and ''Drosophila''.
Mirtrons are the type of microRNAs which are located in the introns of the mRNA encoding host genes. All the miRNAs in plants are derived from the sequential DCL1 cleavages from pri-miRNA to give pre-miRNA (or miRNA precursor). But the mirtrons bypass the DCL1 cleavage and enter as pre-miRNA in the miRNA maturation pathway.
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