Figure 3. miRNA Processing and Activity.
miRNAs are initially transcribed by RNA Polymerase II and expressed as a part of primary miRNAs (pri-miRNAs). The miRNA portion of the pri-miRNA transcript likely forms a hairpin with signals for dsRNA-specific nuclease cleavage. The dsRNA-specific ribonuclease Drosha digests the pri-miRNA in the nucleus to release hairpin that is called pre-miRNA (approximately 70 nt). Exportin-5 exports pre-miRNAs from the nucleus to the cytoplasm, where Dicer cleaves the pre-miRNA approximately 19 bp from the Drosha cut giving a mature miRNA. Each mature miRNA is complementary to a part of one or more messenger RNAs (mRNAs). The annealing of the miRNA to the mRNA(s) inhibits protein translation.