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 microRNA precursors have a stem-loop structure. © Opabinia regalis /Licence Creative Commons