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Introduction:
RNA interference, also known as RNA mediated interference or in short RNAi, is a mechanism of regulation of gene expression by certain RNA molecules such as miRNAs (micro RNAs), siRNAs (short interfering RNA/silencing RNA) etc. MicroRNAs are small single stranded RNA molecules of about 20-24 nucleotides in length and regulate the gene expression (usually silencing) of other genes. These RNAs are first formed from the noncoding RNA genes, in the form of long primary transcripts (pri-miRNA) that are processed to short nucleotide stem loop structures of about 70 nucleotides called pre-miRNA by microprocessor protein complexes (nuclease Drosha + Pasha) in the nucleus. These pre-miRNA molecules are then processed by endonuclease dicer (RNAse III ribonuclease) into a short lived miRNA:miRNA duplex, which is picked up the by the RISC (RNA induced silencing complex; degrades one strand from the miRNA duplex) to form highly specific miRISC complex that can down regulate the gene expression by two different mechanisms; either miRISC degrades the mRNA (if the miRNA and its corresponding mRNA have perfect complementarity) or repression of translation (if the miRNA and its corresponding mRNA have partial complementarity).

SiRNAs are short, double stranded RNA molecules that range from 20-25 nucleotides in length, involved in RNA interference pathway, where they interfere with the expression of some specific genes. The typical structure of these SiRNAs shows a 2 nucleotide overhang at the 3' end that is thought to be generated by the dicer proteins while processing the long double stranded RNA molecules into these short double stranded siRNA molecules. The difference between the siRNAs and the miRNAs is that the former molecules are double stranded unlike those latter molecules which are single stranded.

Role of RNA interference in regulation of biological phenomenon:

The mechanism of RNA interference has been observed to play a vital role in many biological phenomenons such as maintenance of genome structure and organization, repression of gene expression, immunity (genome defense mechanism) etc.
RNA interference in genome maintenance: In many eukaryotic systems, the components of RNA interference pathway are used for the maintenance of genome struncture and organization. In Schizosaccharomyces pombe, it was observed that the RNAi machinery is essential for the heterochromatin formation at the mating type locus and for the silencing of the centromeric repeats,

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