RNA interference (RNAi) was discovered in the experiment of Caenorhabditis elegans by Andrew Z. Fire and Craig C. Mello. The mechanism is double-stranded RNA(dsRNA) cut by the protein Dicer to produce siRNA(small interfering RNA), a 21~23bp short-segment double-stranded RNA with a symmetric 2nt overhang at the 3′-end and a 5′-phosphate and 3′-hydroxy group. This siRNA will be unwound to guide strand and passenger strand, guide strand bind with RISC to make a RISC/siRNA complex. Then this RISC/siRNA complex recognizes and degrades the target mRNA which is siRNA-homologous.
In the process of gene silencing, miRNA has the similar mechanism as siRNA or miRNA can mediate gene suppression by inhibiting translation of target mRNA. Recently discovered miRNA functions include control of cell proliferation, cell death, and fat metabolism in flies, neuronal patteming in nematodes, modulation of hematopoieticlineage differentiation in mammals, and control of leaf and flower development in plants.
RNAi（picture source：Nature Reviews Drug Discovery）
siRNA (small interfering RNA), a short (usually 21 to 23-bp) double-stranded RNA (dsRNA) with phosphorylated 5′ ends and hydroxylated 3′ ends with two overhanging nucleotides, plays an important roal in RNAi owning to its ability to silence target genes. It has established its reputation in many fields including transposition, stem cell maintenance, defence against viruses, tumor therapy and tissue regeneration.
miRNA(MicroRNA) is endogenous 19~30nt RNA that can play important regulatory roles in animales and plants by targeting mRNAs for cleavage or translational repression. miRNAs comprise one of the more abundant classes of gene regulatory molecules in multicellular organisms and likely influence the output of many protein-coding genes.