Figure 1. Functional stages of gene silencing with double-stranded RNA (dsRNA) in cells of lower animals. The figure shows steps involved in local and systemic gene
silencing. Exogenous dsRNA is imported into cells, processed by dicer into small interfering RNA (siRNA; 21 bp + 2-base 30 extensions on each strand) and assembled with
the argonaute protein into the RNA-induced silencing complex (RISC). The RISC complex targets and degrades specific mRNAs based on the siRNA sequence. Systemic
RNAi effects are mediated through the production of new dsRNAs by RNA-dependent RNA polymerase (RdRP), which uses the target RNA as a template and is primed by
siRNA strands. The secondary dsRNAs can be exported from the cell to spread the RNAi effect to other cells. Gene names in italics have been identified in Drosophila
melanogaster. The transport proteins SID-1 and SID-2 have been identified in Caenorhabditis elegans, as has the RdRP enzyme. Transport mechanisms might differ
between diffeent organisms.