Figure 2.

MiRNA biogenesis. A miRNA gene is transcribed by RNA polymerase II. The resultant primary transcript is termed the pri-miRNA. In the nucleus the pri-miRNAs is trimmed into a 60-100 nucleotide pre-miRNA by Drosha in the microprocessor complex. Bound by exportin 5 it is then exported via RanGTP into the cytoplasm. In the cytoplasm the nuclease Dicer further trims the pre-miRNA into a mature 22 nucleotide miRNA which becomes incorporated into a miRISC complex that retains the “guide strand”. The guide strand, perhaps through its “seed” sequence, then recognised the intended target sites frequently but not exclusively located in the 3' UTR of target mRNAs. Target mRNAs often have multiple target sites for any given miRNA; and a given miRNA may target several mRNAs.