Question

In your own words, briefly describe the process and mechanism of RNAi -- starting with double-stranded RNA (dsRNA) and ending with the destruction of mRNA. Include in your answer the roles of ds-RNA, dicer, siRNA, RISC, slicer, and RdRP.

Answer 1

RNA interference (RNAi)-(known as-co-suppressionpost-transcriptional gene silencing), RNA molecules inhibit gene expression or translation, by neutralizing targeted mRNA molecules.RNA interference has an important role in defending cells against parasitic nucleotidesequences – viruses and transposons.In a wide variety of organisms, including animals, plants, and fungi, RNAi is triggered by double-stranded RNA (dsRNA).This mechanism allows the organism to interfere with the foreign genetic sequence before it is able to integrate itself into the host genome and cause damage.

RNA interference (RNAi) process start with the presence of a Double-Stranded RNA (dsRNA) molecule in a cell. This dsRNA can either be exogenous (introduced to the organism coming from infection by a virus with an RNA genome or laboratory manipulations) or endogenous (produced by the cell itself).The presence of dsRNA in cell, it activates a protein endoribonuclease (the ribonuclease protein) called Dicer. Using ATP, the dicer protein is able to recognise the dsRNA and bind tightly to it. Then actively cleaves the long strand of dsRNA into smaller segments made up of 20-25 base pairs with overhanging ends. Dicer binds and cleaves double-stranded RNAs (dsRNAs) in plants, or short hairpin RNAs (shRNAs),These short double-stranded fragments are called small interfering RNAs (siRNAs). These siRNAs are then separated into single strands One strand of the siRNA then binds to an argonaute protein in the cytoplasm, which is part of a multiprotein complex called RNA induced silencing complex RISC, by RISC-Loading Complex (RLC). RISC is then activated when double-stranded siRNA is unzipped with the aid of ATP by the helicase protein of RISC.RLC includes Dicer-2 and R2D2. TATA-binding protein-associated factor 11 (TAF11) assembles the RLC by facilitating Dcr-2-R2D2 tetramerization, which increases the binding affinity to siRNA by 10-fold. Add TAF11 would convert the R2-D2-Initiator (RDI) complex into the RLC. The strand chosen for binding to RISC is the strand with the least thermodynamic stability on the 5’ end. RISC binds to its complementary mRNA.If there is extensive binding to the mRNA, the argonaute component of RISC acts as an endonuclease and cleaves the target mRNA into two. The RISC complex is then released from the strand with the help of ATP, and other cytoplasmic proteins rapidly degrade the cleaved mRNA preventing its translation into a protein effectively silencing the gene. 'secondary' siRNAs are structurally distinct from dicer-produced siRNAs and appear to be produced by an RNA-dependent RNA polymerase (RdRP).