Figure 4. Biogenesis of miRNAs and Assembly into miRISC in Plants and Animals.
Nuclear transcription leads to capped and polyadenylated pri-miRNAs. In plants, Dcl1 processes the RNA in succession. The order of processing is not certain. The terminal loop may first be excised or it might be the flanking segments that are cleaved first. The second processing step by Dcl1 yields a mature miRNA/miRNA* duplex that becomes methylated and exported from the nucleus. In animals, the pri-miRNA is processed by Drosha with the aid of DGCR8 to generate a pre-miRNA species. This is exported from the nucleus and processed by Dicer to form the mature miRNA/miRNA* duplex. After processing, miRNAs are assembled into miRISC. Only one strand of the duplex is stably associated with an miRISC complex—the miRNA strand is usually more strongly favored than the miRISC* strand, although there are exceptions.