MicroRNAs
micrornas
A microRNA (abbr. miRNA) is a small non-coding RNA molecule (ca. 22 nucleotides ) found in plants, animals, and some viruses, which functions in transcriptional and post-transcriptional regulation of gene expression. Encoded by eukaryotic nuclear DNA in plants and animals and by viral DNA in certain viruses whose genome is based on DNA, miRNAs function via base-pairing with complementary sequences within mRNA molecules. As a result, these mRNA strands are silenced because they can no longer be translated into proteins by ribosomes, and such complexes are often actively disassembled by the cell ('target degradation'). The human genome may encode over 1000 miRNAs, which may target about 60% of mammalian genes and are abundant in many human cell types.