Introduction. Mature microRNAs (miRNAs) are a class of naturally occurring, small non-coding RNA molecules, about 21â25 nucleotides in length.icroRNAs are transcribed by RNA polymerase II as large RNA precursors called pri-miRNAs and comprise of a 5' cap and poly-A tail. The pri-miRNAs are processed in the nucleus by the microprocessor complex, consisting of the RNase III enzyme Drosha, and the double-stranded-RNA-binding protein, Pasha/DGCR8.