MicroRNA

Micro ribonucleic acid (microRNA, miRNA, μRNA) are small, single-stranded, non-coding RNA molecules containing 21–23 nucleotides. Found in plants, animals, and even some viruses, microRNAs are involved in RNA silencing and post-transcriptional regulation of gene expression. miRNAs base-pair to complementary sequences in messenger RNA (mRNA) molecules, then silence these mRNA molecules by one or more of the following processes:

Cleaving the mRNA strand into two pieces.

Destabilizing the mRNA by shortening its poly(A) tail.

Reducing translation of the mRNA into proteins.

In cells of humans and other animals, miRNAs primarily act by destabilizing the mRNA.

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 stem-loops (hairpins), whereas siRNAs derive from longer regions of double-stranded RNA. The human genome may encode over 1900 miRNAs. However, only about 500 human miRNAs represent bona fide miRNAs in the manually curated miRNA gene database MirGeneDB.

miRNAs are abundant in many mammalian cell types. They 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.