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A small RNA molecule that is encoded by a cell and can ‘silence’ the expression of a particular target gene within the cell (see RNA interference). miRNAs bind to target messenger RNA (mRNA) molecules and suppress translation of the mRNA into protein. They regulate expression of perhaps a third of all protein-coding genes and are involved in many aspects of embryological development, cell differentiation, cell death, and cancer. It is estimated that more than 2% of genes in animals encode miRNAs, which are transcribed as longer precursors (primary miRNAs, or pri-miRNAs) from which a protein called Drosha cuts a 70–80-nucleotide region. This forms a double-stranded (ds) hairpin structure (the pre-miRNA), which is trimmed by another protein, Dicer, to produce the miRNA – a single RNA strand approximately 21 nucleotides long. This is then incorporated into a complex of proteins called RISC (RNA-induced silencing complex). The miRNA binds to one or more sites with a more-or-less complementary base sequence on the target mRNA, thereby blocking translation of the mRNA in some way. Plant cells seem to lack an equivalent of Drosha, and Dicer is able to trim miRNAs directly from long dsRNA precursors. Compare short interfering RNA.

Subjects: Biological Sciences.

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