rna interference

[prateeiik]

It is said that in the arabidopsis plant the dna is modified so as to produce double stranded rna due to both strands getting transcripted. This in turn binds(being complementary) with the mRna produced by the pathogen, stopping its further translation. Then it it is also said that the source of this complementary rna is a viral genome/transposons. How does this fit into the scenario?? Isnt the source of rna -the modified dna which forms dsRna ?

Can anyone please elaborate on this example and correct me if i am wrong!

[MrMistery]

the cell has no double stranded RNA, and thus any double stranded RNA must come from a virus - that is one of the reasons RNAi mechanisms exist.
Cells do produce a type of RNA whose specific function is to silence genes. It is called microRNA, and what it does it form a loop back onto itself so that the Dicer enzyme can cut it up and then the genes can be silenced. Careful, however, miRNAs are NOT double stranded.

I hope this helps, i don't really get your question

[prateeiik]

then why is the dna of the host modified to produce double stranded rna, in most books its written that along with the template strand even the coding strand undergoes transcription which will form two rna which will inturn bind with each other to form a double stranded rna which ultimately silences the mrna of the pathogen.
is this explanation right, i have checked up a lot of books!

[jonmoulton]

MrMistery's explanation matches my understanding of the RNAi system. VIral RNA may be manufactured as two independent complementary strands by using the vial RNA-directed RNA polymerases. However, when RNAi is produced from animal genomes, the double-stranded RNA is manufactured by cleaving a hairpin twice.

In animals, the RNA is transcribed as a single strand of RNA which contains a self-complementary region, and after the self-complementary region has been transcribed the two complementary sequences anneal together to form the hairpin. In the nucleus, the hairpin is cleaved by Drosha releasing a stem-loop. The hairpin is exported into the cytoplasm and then is cleaved by Dicer, producing a short double-stranded RNA fragment which is loaded onto the RISC complex. When the double strand is loaded, argonaute cleaves one strand; the remaining fragments have too little affinity to remain bound stably to the intact strand and they diffuse away, leafing a single RNA strand bound to RISC. I have mainly read about RNAi systems in animals. Plants might do this somewhat differently.

[MrMistery]

as I understand the system, the only difference in plants and animals is that in plants there can be an amplification step: after the RNA is exported into the cytoplasm, there can be an amplification of the signal by copying this RNA by an endogenous RNA-dependent RNA polymerase, which will produce even more RNA molecules that are substrates for Dicer. But I think the actual RNAi enzymes are pretty much the same..