RNA interference (RNAi) or double-stranded RNA (dsRNA) is a system within living cells that help control which genes are active and how active they are. siRNAs were first discovered by David Baulcombe's group in Norwich, England, as part of post-transcriptional gene silencing (PTGS) in plants1 and subsequently independently identified in a wide variety of eukaryotic organisms. These dsRNAs are rapidly processed into short RNA duplexes of 21 to 28 nucleotides in length, which then guide the recognition and ultimately complement the division of single-stranded RNA, including messenger RNA or viral genomic / antigenomic RNA (Fig . 1). According to their source or function, naturally small RNA have been described: short interfering RNA (siRNA), repeat-associated short interfering RNA (siRNA rasiRNA o) and microRNA (miRNA). RNA interference has many biological functions - is a vital part of the immune response against viruses and also reduces the expression of genes through transcriptional silencing of genes upregulated or promoted by the activation of RNA. Finally, the artificial introduction of long dsRNA or siRNA has been adopted as a tool to inactivate gene expression in both cultured cells and in living organisms.
A biochemical understanding of RNAi pathway is essential to realize that dsRNAs less than 30 base pairs (bp) could be used to trigger an RNAi response in mammals. Tuschl and colleagues demonstrated that transfection of mammalian cells with short RNAs can induce sequence-specific RNAi pathway and therefore exceeded the barrier to the use of RNAi as a genetic tool mammals2. The impetus for using siRNAs and other small RNAs in mammalian cells also came from the long view that the receptor protein kinase (PKR) activation3 and similar responses have been triggered by short dsRNAs effectively. After initial reports, it took a very short period of time to siRNAs triggers to be adopted as a standard component of the toolbox of molecular biology. siRNAs can be introduced into mammalian cells using a variety of standard transfection methods. The intensity and duration of the silencing response is determined by several factors: a population basis, the silencing response is affected mainly by the overall efficiency of transfection, which can be addressed by optimizing the conditions. In each cell, silencing depends on the amount of siRNA that is delivered and the potential of each siRNA to suppress its target, or its power. Even a relatively powerless siRNA can silence its target, provided that sufficient amounts of siRNA were delivered. However, essentially "forcing" the system by providing large amounts of reagent can give rise to many undesirable effects.
A biochemical understanding of RNAi pathway is essential to realize that dsRNAs less than 30 base pairs (bp) could be used to trigger an RNAi response in mammals. Tuschl and colleagues demonstrated that transfection of mammalian cells with short RNAs can induce sequence-specific RNAi pathway and therefore exceeded the barrier to the use of RNAi as a genetic tool mammals2. The impetus for using siRNAs and other small RNAs in mammalian cells also came from the long view that the receptor protein kinase (PKR) activation3 and similar responses have been triggered by short dsRNAs effectively. After initial reports, it took a very short period of time to siRNAs triggers to be adopted as a standard component of the toolbox of molecular biology. siRNAs can be introduced into mammalian cells using a variety of standard transfection methods. The intensity and duration of the silencing response is determined by several factors: a population basis, the silencing response is affected mainly by the overall efficiency of transfection, which can be addressed by optimizing the conditions. In each cell, silencing depends on the amount of siRNA that is delivered and the potential of each siRNA to suppress its target, or its power. Even a relatively powerless siRNA can silence its target, provided that sufficient amounts of siRNA were delivered. However, essentially "forcing" the system by providing large amounts of reagent can give rise to many undesirable effects.
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