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Mechanism of RNAi

Mechanism of RNAi
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23 Iunie, 2021

Exogenous genes such as viral genes, artificially transferred genes, and transposons are randomly integrated into the host cell genome, and when the host cells are used for transcription, some dsRNA is often produced. The host cell responds quickly to these dsRNAs,


Dicer, an endonuclease in its cytoplasm, cuts dsRNA into multiple small pieces of RNA (approximately 21 to 23 bp) with a specific length and structure, that is, siRNA. siRNA is melted into sense strand and antisense strand under the action of RNA helicase in the cell, and then antisense siRNA is combined with some enzymes in the body (including endonuclease, exonuclease, helicase, etc.) to form RNA-induced Silencing complex (RNA-induced silencing complex, RISC). RISC specifically binds to homologous regions of mRNA expressed by exogenous genes. RISC has the function of nuclease, which cuts mRNA at the binding site, and the cleavage site is the two ends that are complementary to the antisense strand of siRNA. The cleaved fragmented mRNA is then degraded, thereby inducing the host cell's degradation response to these mRNAs. siRNA can not only guide RISC to cut homologous single-stranded mRNA, but also can be used as a primer to bind to target RNA and synthesize more new dsRNA under the action of RNA-dependent RNA polymerase (RdRP). The newly synthesized dsRNA is then used by Dicer The cleavage produces a large amount of secondary siRNA, which further amplifies the role of RNAi, and finally degrades the target mRNA completely.

RNAi occurs in all eukaryotic cells except prokaryotes. It should be noted that, because dsRNA has the potential to inhibit gene expression, any situation that leads to the formation of dsRNA in normal organisms will cause undesired corresponding gene silence. Therefore, the effective expression of various genes in the normal body has a strict mechanism to prevent the formation of dsRNA.

Researchers such as Benjamin Lewis of the Whitehead Institute for Biomedical Research found that small RNAs can regulate gene expression by blocking protein synthesis. They used a computational model to determine small RNAs and corresponding genes, and found evidence that miRNAs control a large part of life functions.

The researchers compared the genomes of humans and dogs, chickens, and mice, and sought correspondences between the protein synthesis genes shared by these species and miRNAs. It was found that although these species began to "separate" and evolve independently 310 million years ago, the genes regulated by miRNAs in their genomes accounted for about one-third, and these genes were preserved during the evolutionary process. No changes have occurred. Lewis said that with the release of more genome data and the advancement of experimental technology, it is possible to discover that more genes are regulated by small RNAs.

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