Model for miRNA biogenesis and real time PCR analysis of the five key enzymes implicated in miRNA biogenesis. a: Model for miRNA biogenesis. miRNA genes are transcribed by Pol II to generate the primary transcripts (pri-miRNAs). The initiation step (cropping) is mediated by the Drosha-DGCR8 complex. The product of the nuclear processing is ~ 70-nt pre-miRNA. This structure can serve as a signature motif that is recognized by the nuclear export factor Exportin-5. Pre-miRNA constitutes a transport complex together with Exportin-5 and its cofactors. Upon export, the cytoplasmic RNase III Dicer participates in the second processing step (dicing) to produce miRNA duplexes (~ 22-nt). The duplex is recognized by the PAZ domain of the Ago protein and incorporated into RISC. Usually one strand is selected as the mature miRNA, whereas the other strand is degraded. b: Real time PCR analyses of the expression levels of DGCR8, Drosha, Exportin-5, Dicer and Ago in DVC and HIP in rats (n = 5). The value for DVC was set as one.