<img alt="植物microRNAs合成過程" 植物micrornas合成"="" border="1" height="511" data-cke-saved-src="http://www.bio1000。。ｃｏｍ/uploads/allimg/120427/1A0044093-0.jpg" src="http://www.bio1000。。ｃｏｍ/uploads/allimg/120427/1A0044093-0.jpg" width="494" style="vertical-align: middle; border: 0px;">

microRNAs (miRNAs) constitute a large group of endogenous single stranded small RNAs (19-24 nucleotides) having negative regulatory function on gene expression. They are processed from longer non-coding RNAs with characteristic fold-back structures by double-strand specific RNases of the Dicer family. Upon processing, they are incorporated in the RNA induced silencing complex RISC by binding to its main component, an Argonaute protein. miRNAs serve as the specificity components of RISC, since they base-pair to target nucleic acids, mostly mRNAs, in the cytoplasm. Subsequent regulatory events include target mRNA cleavage and destruction, which is mainly triggered by plant miRNAs, and translational inhibition, as mostly seen in animals. Target sites in plant miRNAs normally share high sequence complementarity to the respective miRNA and are often in coding regions, whereas in animals, target sites are often only partially complementary to their miRNAs and are mostly located in the 3'UTR of target GENEs.