Abstract
Plants and animals have single-stranded silencing RNAs (sRNAs) of 21–25 nucleotides in length that are derived from a double-stranded (ds)RNA precursor by Dicer (DCL) processing. These RNAs are the guide RNA for nucleases of the AGO classthat cleave targeted RNA in a nucleotide sequence-specific manner. The cleaved RNAs are then degraded further or they arethe template for an RNA-dependent RNA polymerase (RDR) that generates a dsRNA. In this paper, I discuss the possibilitythat this RDR-generated dsRNA initiates a cascade in which there are multiple rounds of secondary sRNA production. I proposethat these secondary sRNAs feature in mechanisms that can either buffer mRNA populations against change or, in certaincircumstances, mediate extensive changes in mRNA populations. The RNA cascades may also have RNA-mediatedepigenetic characteristics in addition to the DNA and chromatin transcriptional silencing potential that has been previouslylinked with RNA silencing.