The Science of RNAi

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The RNAi pathway may have originally been an early virus-identification system in the cell, identifying and destroying any double-stranded RNA as an effort to prevent viral replication. An alternate hypothesis is that the RNAi pathway affects transcription in some way, as small RNAs may determine which genes are transcribed and which remain wrapped inside the chromatin.

The RNAi mechanism of action is triggered when short double-stranded RNA (dsRNA) is formed. Double stranded RNA can occur several ways:

This double-stranded RNA forms, with several proteins, the RNA-induced silencing complex (RISC), which utilizes the dsRNA as a template to destroy any RNA that shares the sequence of the dsRNA. The RISC is extremely effective at silencing specific genes without affecting other genes.

The RNAi response has been demonstrated effectively in mammalian cells and tissues for both exogenous and endogenous genes. The knockdown effect can be partial, diminishing but not completely silencing the expression of a gene, or complete, turning off the function of a gene entirely. Long dsRNA (>30 base pairs) can trigger the interferon response, leading to general mRNA cleavage and apoptosis, making length one of the crucial parameters to control in the manufacture of RNAi products.

Discovery and explanation of the RNAi effect won Andrew Fire and Craig Mello the Nobel Prize in Medicine in 2006. They initially injected nematodes (the first multicellular organism to have a fully-sequenced genome) with a single strand of RNA that bound a target mRNA, later discovering that injecting a double-stranded version of the same sequence amplified the observed gene-silencing effect.

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