Dicer activity

Speculate about why plants deficient In Dicer activity show Increased sensitivity to Infection by RNA viruses. 

Fig. 22.1. A model of gene silencing. Long, double-stranded RNAs (dsRNAs) are processed into 20-26 nucleotide small interfering RNAs (siRNAs) by Dicer (Step 1). The siRNAs associate with an RNA-induced silencing complex (RISC, Step 2), unwinding and activating in the process (Step 3). The antisense strand of the siRNA guides the RISC to complementary mRNA molecules (Step 4), which are cleaved and destroyed (Step 5). Sense and antisense RNA strands are indicated by thick and thin lines, respectively.

Fig. 3.2 (A) The RNAi pathway Dicer with its RNAse III domains cleaves long double-stranded RNA (dsRNA) twice per strand, and generates the siRNA duplex. After cleavage, a siRNA/protein complex (siRNP) is formed. In the next step, the siRNA is unwound and the RNA-induced silencing complex (RISC) becomes activated. The target...

The various forms of electron microscopy (EM) have been used to examine structural features of nucleic acid-protein interactions. EM structures include protein interactions with the ribosome, an activator-dependent transcription initiation complex, the core editing complex in trypanosomatid mitochondria, human Dicer from the RISC complex showing that it has an L-shaped structure, the RNA editing complex from trypanosomes and loading of minichromosome maintenance proteins (Mcm2-7) onto DNA during DNA replication. Scanning electrochemical microscopy (SECM) with DNA attached to an electrode has been used to detect an A-C mismatch site, and to visualise the activity of an enzyme to detect hybridisation. " Cryo-EM (CEM) has also been... 

Fig. 1. Model of asymmetric RISC activation. RNAi transgenes produce shRNA molecules as shown. The strand antisense to the target mRNA is black, and the sense strand is gray. Dicer processes shRNA to siRNA, a duplex with 5 phosphorylated ends and 2-nt 3 overhangs. RISC may interact with siRNA at either end, but incorporates the strand it reads only in a 5 to 3 direction. For effective RNAi, RISC should be biased to acquire the antisense strand. This can be accomplished by designing the 5 end of the antisense strand to be A/U rich, while simultaneously making the other end of the siRNA molecule G/C rich. This tips the balance toward preferential uptake of the antisense strand and results in knockdown of the targeted gene.

FIGURE 15.2 The mechanism of RNA interference (RNAi). Exogenous double stranded RNA or endogenous hairpin RNA is cleaved by Dicer to make short interfering RNAs (siRNAs). These are incorporated into the RNA-induced silencing complex (RISC). Upon activation with ATP, the siRNA unwinds and the RISC complex mediates the cleavage of the target mRNA. 

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