RNAi was first described and named so by molecular biologists Andrew Fire of the Carnegie Institute of Washington and Craig Mello of the University of Massachusetts, along with their colleagues, in a landmark 19 February 1998 Nature paper.In RNAi ,double stranded RNA(ds RNA) is cleaved invivo by Dicer Ribonuclease into 21-23 nucleotide small interfering RNA(si RNA) which in association with RNA-induced silencing complex (RISC) guides sequence specific mRNA degradation.RNAi mechanism is observed in many organisms thought to be derived from ancient mechanism to combat viral infections.
Experimentally introduced dsRNA can trigger the RNAi effect in a wide range of experimental organisms, providing a powerful method for disrupting gene function.In Drosophila Double-stranded RNA (dsRNA) that is homologous to the target gene is delivered by two ways:
1) Injection of in vitro-transcribed RNA into individual embryos .
2) Expression of an inverted repeat RNA in vivo. This can be achieved by GAL4 UAS system by cloning unique sequence of the gene of interest in RNAi vector for generating hair pin structure or double stranded RNA.A main advantage of RNAi by expression is the possibility of interfering with gene function in specific tissues and at specific time during development.