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Fig. 2. Biogenesis and mode of action of miRNAs. miRNA biogenesis in
(A) animals and (B) plants. The red miRNA strand is the strand
incorporated into the Ago effector complex. The blue miRNA strand, referred to
as miRNA*, becomes degraded. Drosha acts as the RNase III in some
animal nuclei, and nuclear Dicer as the RNase III in the plant nucleus, where
it cleaves the pri-miRNA in two steps (1,2). The cytoplasmic RNase III in
animals is Dicer. RNAse III enzymes usually partner with distinct
double-stranded RNA-binding-domain-containing proteins (dsRBPs, in gold) in
the nucleus. Following their export from the nucleus, miRNAs then associate
with Ago. In animals, the AGO-containing miRNPs predominantly associate with
GW182, a protein with glycine-tryptophan (GW) repeats that is required for P
body integrity. The miRNA subsequently translationally represses its target
and is then localized to P bodies. In plants, miRNAs predominantly function
through target mRNA cleavage, which can also occur in animals (see text for
more details). m7G, 5' methyl(7)G cap of target mRNA; me,
2'-O-methyl group on the 3' end of the RNA; miRNP, effector
ribonucleoprotein complex that mediates translational repression or target
mRNA cleavage directed by miRNAs; p, 5' phosphate group.
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