Fig 1.

Biogenesis of miRNA and sites of regulation. miRNA is transcribed by RNA polymerase II to produce the primary precursor miRNA (I, pri-miRNA). The pri-miRNA is trimmed by Drosha/DGCR8 in the nucleus to produce the ∼ 70 nt precursor miRNA (II, pre-miRNA). The pre-miRNA is transported to the cytoplasm by Exportin 5 where it interacts with the RNAse III Dicer and TRBP/ Loquacious to produce the ∼ 21nt miRNA: miRNA* duplex (III). The miRNA strand (IV) is loaded into RNA-induced silencing complex (RISC) and the miRNA* strand is typically degraded. Possible sites of regulation include: A, A-I editing; B, SNPs or mutations; C, regulation of processing by miRNA-specific factors; D, altered levels of Drosha; E, altered levels of Dicer; F, sequestration into subcellular organelles.