Background:
MicroRNAs are small non-coding RNAs, usually transcribed from larger genes coding for other mRNAs. They range from 17-22 nucleotides in length and act as an endogenous RNA interference mechanism.
microRNA processing:
microRNAs are processed in the nucleus by Drosha then exported out into the cytoplasm by exportin-5. The hairpin of the miRNA is then cleaved off by Dicer. microRNAs are unwound to produce single-stranded RNA, which are loaded onto the RNA-induced silencing complex (RISC). RISC facilitates recognition of complementary strands of RNA in the cell, which are then cleaved, inhibiting translation of these mRNAs into proteins. Very similar to the action of exogenously introduced shRNA, but carried out endogenously in each cell on a regular basis... neat!