Fig. 1.

Biogenesis of small RNAs. A. In the miRNA pathway, primary transcripts containing stem-loop structures are first processed by Drosha and then by Dicer to generate mature miRNAs. B (I). The production of siRNAs from dsRNA duplexes or long hairpin structures only requires processing by Dicer. B (II). In addition to initial siRNA biogenesis, the production of siRNA can be amplified by RdRP. The primary siRNA (red) in complex with the AGO protein recruits RdRP. RdRP then uses the target mRNA as a template to generate pools of secondary siRNA (green). C. The production of piRNAs involves cleaving single-stranded (ss) primary transcripts by Zucchini. These leaving behind truncated RNA fragments containing a 5′ Uridine that are then trimmed into mature piRNAs by exonucleases.