Fig. 1 

Canonical and non-canonical pathways of miRNA biogenesis. A Canonical pathway of miRNA biogenesis. Primary transcripts (pri-miRNAs) are transcribed from miRNA genes by RNA polymerase II and are then cleaved by Microprocessor (Drosha+DGCR8) to form precursor miRNAs (pre-miRNAs). Pre-miRNAs are then exported to the cytoplasm, where they are once again cleaved by Dicer, forming miRNA duplexes. One strand of this miRNA duplex is loaded into Argonaute protein (AGO) to form RISC, which takes part in mRNA regulation. B Non-canonical pathway of miRNA biogenesis. During the non-canonical pathway of miRNA biogenesis, miRNAs bypass the Drosha- or Dicer-dependent cleavage step, and this step is replaced by another cleavage reaction carried out by different proteins