Schematic presentation of the second step of the RNAi pathway. RNAi is a process that generates short interfering RNA (siRNA) which directs an endonuclease to mRNA with complementary sequence and silences gene expression post-transcriptionally (cleavage). The cartoon depicts assembly of the "slicer" complex from its individual constituents, believed to contain helicase (orange) and endonuclease (green) activities that target mRNA for degradation. The slicer complex is shown assembling with siRNA, unwinding the duplex siRNA allowing the antisense strand to pair with the mRNA and direct an endonuclease to a precise sequence on the mRNA as it emerges from the nucleus. It is unknown whether the slicer complex disassembles to its individual components and reassembles with a new siRNA (stoichiometric relationship between siRNA-complex and target mRNA) or if the slicer complex retains the antisense strand of the to target another mRNA (catalytic relationship between siRNA-complex and target mRNA).

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