Fig. 3.

Biogenesis of noncoding RNA. Double stranded RNA are the major precursors which may be produced by inverted repeats in DNA, or two different RNA having complementary sequences may be simultaneously transcribed, or replication of double stranded RNA virus. Steps in biogenesis include (1) These may be formed in introns of pre-mRNA or miRNA genes having own promoters (transcribed by RNA polymerase II as large pri-miRNA hair pin loop like structure. This is recognised by DGCR8 (DiGeorge syndrome chromosomal region 8) and in association with Drosha forms Micro Processor Complex which cuts the RNA into smaller precursor microRNA. (2) This is then exported out of the nucleus into the cytoplasm with EXPORTIN-5 molecule. (3) In cytoplasm it is recognised by DICER helicase with RNase motif which cleaves the stem loop and releases microRNA molecule.4. Argonaute protein AGO-2 associates with miRNA/DICER, unwinds the double stranded RNA and releases one of the starnds. 5. It retains one of the strands known as Guide strand and other proteins now associate with it forming RISC (RNA Induced Silencing Complex) which guides this complex to the target complementary mRNA either by degrading mRNA or by inhibiting translation by preventing ribosomes from binding