MicroRNA
MicroRNAs (miRNAs) are a class of small RNAs, which are 22 nucleotides long. These RNAs play an important part in regulation of translation and degradation of mRNAs. The regulation is accomplished by complementary fixation of miRNAs with partially complementary sites in untranslated regions (UTRs) of mRNAs (targets).
Like all RNAs, miRNAs are encoded by genes. They were first discovered in the year 2001 in Caenorhabditis elegans. At the present time miRNAs are found in animals and plants. Despite the impressive number of miRNAs, discovered in animals (about 500), not so many targets are found. It was shown, that the expression of miRNAs in animals includes two stages. First, an extended full-length transcript, which can contain more than one copy of miRNAs, is transcribed from the miRNA gene. Processing of the full-length transcript is accomplished in the nucleus, and the product of processing (a hairpinned precursor) is exported into the cytoplasm. In the cytoplasm, Dicer enzyme (which resembles ribonuclease III) cuts a mature microRNA out of the precursor.