By Gertrud U. Rey
Post-transcriptional gene silencing (PTGS) is an RNA regulation pathway in eukaryotes that depends on the presence of double stranded RNA (dsRNA) in the cytoplasm of cells. As stated by Nels Elde in TWiEVO 8, dsRNA in the cytoplasm has an effect synonymous to “blood in the water” for the innate immune system, triggering a cascade of events that leads to the degradation of complementary mRNAs. In plants, this pathway has been thought to serve as the predominant mechanism of antiviral defense.
All immune responses, from intrinsic to adaptive, need to be regulated – if left on indefinitely they will damage the host. The 
Mitochondria are descended from bacteria that invaded cells 1.5 billion years ago and never left. The mitochondrial genome is like that of bacteria: circular double-stranded DNA, only smaller. And just like the genome of bacteria, RNA can be made from both strands of mitochondrial DNA – which results in the formation of dsRNA. Fortunately there are systems in place to make sure that this dsRNA