microRNAs (miRNA or μRNA) are a family of single-stranded
non-coding RNA molecules of 21-23 nucleotides in length. The
discovery of microRNAs added a new layer of gene expression at
post-transcriptional level. The primary transcript of microRNA is a
pri-miRNA, which is processed into a short stem-loop structure
named a pre-miRNA. The final functional form of a microRNA is then
loaded into RISC (RNA-induced silencing comples).
microRNAs regulate their target, mRNAs, by perfect (mostly in
plants) of imperfect (mostly in mammalian cells) complementary
binding. One microRNA can regulate multiple mRNAs by repressing
translation or destablizing mRNAs.