RNA interference

RNA interferenceFrom Wikipedia, the free encyclopedia Jump to: navigation, searchLentiviral Delivery of designed shRNAs and the mechanism of RNA interference in mammalian cells. RNA interference (RNAi) is a system within living cells that takes
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RNA interferenceRNA interferenceFrom Wikipedia, the free encyclopediaJump to: navigation, searchLentiviral Delivery of designed shRNAs and the mechanism of RNA interference inmammalian cells.RNA interference (RNAi) is a system within living cells that takes part in controllingwhich genes are active and how active they are. Two types of small RNA molecules –microRNA (miRNA) and small interfering RNA (siRNA) – are central to RNAinterference. RNAs are the direct products of genes, and these small RNAs can bind tospecific other RNAs and either increase or decrease their activity, for example bypreventing a messenger RNA from producing a protein. RNA interference has animportant role in defending cells against parasitic genes – viruses and transposons – butalso in directing development as well as gene expression in general.The RNAi pathway is found in many eukaryotes including animals and is initiated by theenzyme Dicer, which cleaves long double-stranded RNA (dsRNA) molecules into shortfragments of ~20 nucleotides. The siRNA will be unwound into two ssRNA, namely thepassenger strand and the guide strand. The passenger strand will be degraded, and theguide strand is incorporated into the RNA-induced silencing complex (RISC). The mostwell-studied outcome is post-transcriptional gene silencing, which occurs when the guidestrand base pairs with a complementary sequence of a messenger RNA molecule andinduces cleavage by Argonaute, the catalytic component of the RISC complex. Thisprocess is known to spread systemically throughout the organism despite initially limitedmolar concentrations of siRNA.The selective and robust effect of RNAi on gene expression makes it a valuable researchtool, both in cell culture and in living organisms because synthetic dsRNA introducedinto cells can induce suppression of specific genes of interest. RNAi may also be used forlarge-scale screens that systematically shut down each gene in the cell, which can helpidentify the components necessary for a particular cellular process or an event such ascell division. Exploitation of the pathway is also a promising tool in biotechnology andmedicine.Historically, RNA interference was known by other names, including cosuppression, posttranscriptional gene silencing, and quelling. Only after these apparently unrelatedprocesses were fully understood did it become clear that they all described the RNAiphenomenon. In 2006, Andrew Fire and Craig C. Mello shared the Nobel Prize inPhysiology or Medicine for their work on RNA interference in the nematode worm C.elegans,[1] which they published in 1998.[2]Contents[hide] 1 Cellular mechanism • 1.1 dsRNA cleavage o 1.2 MicroRNA o 1.3 RISC activation and catalysis o 1.4 Transcriptional silencing o 1.5 Crosstalk with RNA editing o 1.6 Variation among organisms o 1.7 Related prokaryotic systems o 2 Biological functions • 2.1 Immunity o 2.2 Downregulation of genes o 2.3 Upregulation of genes o 3 Evolution • 4 Technological applications • 4.1 Gene knockdown o 4.2 Functional genomics o 4.3 Medicine o 4.4 Biotechnology o 5 History and discovery • 6 References • 7 External links • [edit] Cellular mechanismThe dicer protein from Giardia intestinalis, which catalyzes the cleavage of dsRNA tosiRNAs. The RNase domains are colored green, the PAZ domain yellow, the platformdomain red, and the connector helix blue.[3]RNAi is an RNA-dependent gene silencing process that is controlled by the RNA-induced silencing complex (RISC) and is initiated by short double-stranded RNAmolecules in a cells cytoplasm, where they interact with the catalytic RISC componentargonaute.[1] When the dsRNA is exogenous (coming from infection by a virus with anRNA genome or laboratory manipulations), the RNA is imported directly into thecytoplasm and cleaved to short fragments by the enzyme Dicer. The initiating dsRNA canalso be endogenous (originating in the cell), as in pre-microRNAs expressed from RNA-coding genes in the genome. The primary transcripts from such genes are first processedto form the characteristic stem-loop structure of pre-miRNA in the nucleus, then exportedto the cytoplasm to be cleaved by Dicer. Thus, the two dsRNA pathways, exogenous andendogenous, converge at the RISC complex.[4][edit] dsRNA cleavageEndogenous dsRNA initiates RNAi by activating the ribonuclease protein Dicer,[5] whichbinds and cleaves double-stranded RNAs (dsRNAs) to produce double-strandedfragments of 20–25 base pairs with a 2 nucleotide overhang at 3 end.[6][7][8][9]Bioinformatics studies on the genomes of multiple organisms suggest this lengthmaximizes target-gene specificity and minimizes non-specific effects.[10] These shortdouble-stranded fragments are called small interfering RNAs (siRNAs). These siRNAsare then separated into single strands and integrated into an active RISC complex. Afterintegration into the RISC, siRNAs base-pair to their target mRNA and induce cleavage ofthe mRNA, thereby preventing it from being used as a translation template.[11]Exogenous dsRNA is detected and bound by an effector protein, known as RDE-4 in C.elegans and R2D2 in Drosophila, that stimulates dicer activity.[12] This protein only bindslong dsRNAs, but the mechanism producing this length specificity is unknown.[12] TheseRNA-binding proteins then facilitate transfer of cleaved siRNAs to the RISC complex.[13]In C. elegans, this initiation response is amplified by the cell by the s ...