The Rna Interference ( Rnai ) Pathway

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The RNA interference (RNAi) pathway is an important biological system that is utilised by many organisms as a method of regulating gene expression. But what exactly is RNAi? And what is the importance of its use and application in biology?
RNAi is a cellular process that actively silences specific genes. This method of post-translational gene regulation has been observed in many organisms including plants, fungi, some bacteria and animals. RNAi inhibits protein synthesis of the target transcript by the use of small RNA molecules that target messenger RNA (mRNA) within the cytoplasm following transcription by an RNA polymerase. RNAi occurs during translation of the mRNA by inhibiting the action of Ribosomes that catalyse this process within
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Upon binding the two strands of siRNA are separated, with one strand – known as the guide strand - remaining bound to RISC and the other strand being degraded. The guide strand then directs RISC towards its target mRNA for cleavage (Zamore, et al. 2000). This mechanism is extremely precise as it relies on complimentary base pairing between the bound siRNA and its target mRNA. This complementarity allows RISC to bind to the target mRNA where the Argonaute protein catalyses it’s cleavage, resulting in degradation of the target mRNA (Fig. 1).
This mechanism inhibits translation of the protein for which the target mRNA codes for and therefore silences that specific gene. RNAi can also involve the use of coded short hairpin RNAs (shRNAs), which are larger dsRNA molecules containing a loop section which folds back on itself to form a double stranded molecule contains a sense and antisense strain. The shRNAs are cleaved into siRNAs when they bind to RISC and can also found in examples of exogenous introduction such as viral or experimental insertion. The ability to post translational control of gene expression with a precise sequence specific manner has been exploited experimentally in researching gene knockdown and loss of function gene analysis.

In 2006, Andrew Fire and Craig Mello were awarded the Nobel Prize in Physiology or Medicine for their work in RNAi in Caenorhabditis elegens. Their 1998 paper demonstrated RNAi of endogenous
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