 The RNAi pathway is found in many eukaryotes , including animals , and is initiated by the enzyme Dicer , which cleaves long double @-@ stranded RNA ( dsRNA ) molecules into short double @-@ stranded fragments of ~ 20 nucleotide siRNAs . Each siRNA is unwound into two single @-@ stranded RNAs ( <unk> ) , the passenger strand and the guide strand . The passenger strand is degraded and the guide strand is incorporated into the RNA @-@ induced silencing complex ( RISC ) . The most well @-@ studied outcome is post @-@ transcriptional gene silencing , which occurs when the guide strand pairs with a complementary sequence in a messenger RNA molecule and induces cleavage by <unk> , the catalytic component of the RISC complex . In some organisms , this process spreads systemically , despite the initially limited molar concentrations of siRNA . 
