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RNAi - This video shows how RNAi works in the cells.
RNA interference (RNAi) is a mechanism that inhibits gene expression at the stage of translation or by hindering the transcription of specific genes. RNAi targets include RNA from viruses and transposons (significant for some forms of innate immune response), and also plays a role in regulating development and genome maintenance. Small interfering RNA strands (siRNA) are key to the RNAi process, and have complementary nucleotide sequences to the targeted RNA strand. Specific RNAi pathway proteins are guided by the siRNA to the targeted messenger RNA (mRNA), where they "cleave" the target, breaking it down into smaller portions that can no longer be translated into protein. A type of RNA transcribed from the genome itself, microRNA (miRNA), works in the same way. The RNAi pathway is initiated by the enzyme dicer, which cleaves long, dsRNA molecules into short fragments of 21 23 base pairs. One of the two strands of each fragment, known as the guide strand, is then incorporated into the RNA-induced silencing complex (RISC) and pairs with complementary sequences. The most well-studied outcome of this recognition event is post-transcriptional gene silencing. This occurs when the guide strand specifically pairs with an mRNA molecule and induces cleavage by argonaute, the catalytic component of the RISC complex. Another outcome is epigenetic changes to a gene histone modification and DNA methylation affecting the degree the gene is transcribed.The selective and robust effect of RNAi on gene expression makes it a valuable research tool, both in cell culture and in living organisms because synthetic dsRNA introduced into cells can induce suppression of specific genes of interest. RNAi may also be used for large-scale screens that systematically shut down each gene in the cell, which can help identify the components necessary for a particular cellular process or an event such as cell division. Exploitation of the pathway is also a promising tool in biotechnology and medicine.Historically, RNA interference was known by other names, including post transcriptional gene silencing, and quelling. Only after these apparently-unrelated processes were fully understood did it become clear that they all described the RNAi phenomenon. RNAi has also been confused with antisense suppression of gene expression, which does not act catalytically to degrade mRNA, but instead involves single-stranded RNA fragments physically binding to mRNA and blocking protein translation. In 2006, Andrew Fire and Craig C. Mello shared the Nobel Prize in Physiology or Medicine for their work on RNA interference in the nematode worm C. elegans, which they published in 1998.
SOURCE: WIKIPEDIA (http://en.wikipedia.org/wiki/RNAi)