How does miRNA regulate gene expression within a RNAi pathway?

MicroRNAs (miRNA) are single-stranded RNA molecules of about 21?23 nucleotides in length, which regulate gene expression. miRNAs are encoded by genes from whose DNA they are transcribed, but miRNAs are not translated into protein (i.e., they are non-coding RNA); instead each primary transcript (a pri-miRNA) is processed into a short stem-loop structure called a pre-miRNA, and finally into a functional miRNA. Mature miRNA molecules are partially complementary to one or more messenger RNA (mRNA) molecules.  Their main function is to down-regulate gene expression by binding to complementary mRNA, thereby inactivating the mRNA (i.e., inhibiting its translation), and sometimes facilitating cleavage of the mRNA.  miRNAs may also target methylation of genomic sites which correspond to targeted mRNAs. miRNAs function in association with a complement of proteins, collectively termed the miRNP.

miRNA microarrays have been developed which contain all known miRNAs for a given species.  Agilent has commercialized a human miRNA microarray.

The target site of an miRNA on an mRNA can be blocked by a steric blocking oligo.

Dysregulation by miRNA may be the cause of various cancers, as well as heart disease and viral infections.