Courtesy of Bonnie Bartel and David P. Bartel Reprinted from Plant Physiol, 132:1-9, June 2003.
 CURRENT MODEL: (A) A microRNA (MIR) gene encodes a primary transcript with a stem-loop structure. A hairpin precursor (in brackets) has been found in animals but not in plants. The enzyme Dicer cleaves the transcript to form a microRNA (miRNA) which nestles in a ribonucleoprotein (RNP) complex that is similar, if not identical, to the RNA-induced silencing complex (RISC). (B) When an mRNA has near-perfect complementarity to an miRNA, the mRNA is cleaved. (C) When an mRNA has less complementarity to an miRNA, the mRNA's translation is attenuated.

The RNA paradigm is due for another correction. Once, RNA seemed a mere conduit between genes and proteins. Its status expanded with reports of catalytic RNA 20 years ago and of endogenous RNA interference 10 years later. Now microRNAs, the 20- to 24-nucleotide transcripts that underlie much...

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