MicroRNA (miRNA) is composed of approximately twenty nucleotides, and is one of the main regulatory pathways for gene expression.
MicroRNAs are coded by the genome and then transcribed into a precursor in a handle and loop shape (like a tennis racquet). Enzymes cleave the RNA into a small single strand fragment of 21 to 24 nucleotides long.
Once they have matured this way the microRNA can regulate gene expression by binding to messenger RNAs carrying a homologous sequence. These are then damaged or their translation is inhibited.
The microRNAs have also been shown to be able to directly methylate DNA in order to switch off genes.
The microRNA precursors have a stem-loop structure. © Opabinia regalis /Licence Creative Commons