Gene silencing RNA

A microRNA (abbreted miRNA) is a small non-coding RNA molecule (containing about 22 nucleotides) found in plants, animals and some viruses, that functions in RNA silencing and post-transcriptional regulation of gene expression. While the majority of miRNAs are located within the cell, some miRNAs, commonly known as circulating miRNAs or extracellular miRNAs, have also been found in extracellular environment, including various biological fluids and cell culture media. Encoded by eukaryotic nuclear DNA in plants and animals and by viral DNA in ceain viruses whose genome is based on DNA, miRNAs function base-pairing with complementary sequences within mRNA molecules. As a result, these mRNA molecules are silenced, by one or more of the following processes: Cleavage of the mRNA strand into two pieces, Destabilization of the mRNA through shoening of its poly(A) tail, and 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 sho hairpins, whereas siRNAs derive from longer regions of double-stranded RNA. The human genome may encode over 1000 miRNAs, which are abundant in many mammalian cell types[ and appear to target about 60% of the genes of humans and other mammals