Fig. 2

The process of miRNA biogenesis and role in post-transcriptional suppression. The biogenesis of miRNAs commences in the nucleus where the RNA polymerase II transcribes the genetic sequence encoding the miRNA to produce a primary miRNA hairpin (pri-miRNA) which is capped, polyadenylated and has a stem-loop structure. Further processing by the ribonuclease DROSHA enzyme occurs in the nucleus before the resultant 70 to 100 nt long pre-miRNA hairpin is transported to the cytoplasm via the Exportin5 protein (XPO5). Once the double stranded pre-miRNA is in the cytoplasm, RNAse DICER cleaves the molecule into two single strands, with a leading functional strand, and a passenger strand -often referred to as (*)- which will be degraded. The Ago proteins bind to the leading single stranded miRNA to form the RISC. The RISC is considered to be the functional unit in this process which facilitates the binding of miRNA into the targeted mRNA resulting in either translation repression or target degradation. Source: MS Powerpoint