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Concept
MicroRNA
Summary
MicroRNA (miRNA) are small, single-stranded, non-coding RNA molecules containing 21 to 23 nucleotides. Found in plants, animals and some viruses, miRNAs are involved in RNA silencing and post-transcriptional regulation of gene expression. miRNAs base-pair to complementary sequences in mRNA molecules, then gene silence said mRNA molecules by one or more of the following processes:
Cleavage of mRNA strand into two pieces,
Destabilization of mRNA by shortening its poly(A) tail, or
Translation of mRNA into proteins.
This last method of gene silencing is the least efficient of the three, and requires the aid of 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 1900 miRNAs, although more recent analysis suggests that the number is closer to 2,300. However, only about 500 human microRNAs represent bona fide miRNA in the manually curated miRNA gene database MirGeneDB.
miRNAs are abundant in many mammalian cell types and as extracellular circulating miRNAs. Circulating miRNAs are released into body fluids including blood and cerebrospinal fluid and have the potential to be available as biomarkers in a number of diseases. MiRNAs appear to target about 60% of the genes of humans and other mammals. Many miRNAs are evolutionarily conserved, which implies that they have important biological functions. For example, 90 families of miRNAs have been conserved since at least the common ancestor of mammals and fish, and most of these conserved miRNAs have important functions, as shown by studies in which genes for one or more members of a family have been knocked out in mice.
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.
Official source
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