My textbook says that in RNAi, a complementary double stranded RNA sequence attaches to an mRNA and silences it using a protein machinery. I Googled and read about this so now I know what siRNA and RISC and microRNA are. But the next thing my book says is
“the source of these complementary double stranded RNA sequence [obviously my textbook means siRNA] may be viruses and transposable elements.”
What does this mean? Do transposons make siRNA? I thought transposons were non-coding except when they travel via the retrotransposon mechanism.
Do viruses that infect eukaryotic cells donate their own RNA genetic material for the cell to use in RNAi? But I thought RNAi was a cellular defence against viruses, how can viruses help give the dsRNA that’s supposed to fight the viruses? A number of sites online simply state that siRNA is exogenous is origin; do they mean that it comes from viruses?
I would greatly appreciate it if someone could take the time to answer each of my questions one by one or even recommend a site that answers them all.