mikrorNA

MicroRNA (miRNA) are short non-coding RNA molecules, typically 21 to 24 nucleotides in length, that regulate gene expression at the post-transcriptional level. They are transcribed as longer primary transcripts (pri-miRNAs) that are capped and polyadenylated. In animals, pri-miRNAs are processed in the nucleus by the Drosha-DGCR8 complex to form precursor miRNAs (pre-miRNAs), ~70 nucleotides in length with a hairpin structure. Pre-miRNAs are exported to the cytoplasm by Exportin-5 and cleaved by Dicer to generate a duplex of mature miRNA and its star strand. The guide strand is incorporated into an Argonaute-containing RNA-induced silencing complex (RISC), while the passenger strand is typically degraded.

miRNA regulate gene expression primarily by binding to complementary sequences in target messenger RNAs, most often in the 3' untranslated region. The interaction relies on the seed region, nucleotides 2–8 of the miRNA, and typically results in translational repression or mRNA destabilization. Plant miRNAs may show near-perfect complementarity and can direct cleavage.

Biological roles of miRNAs are broad, influencing development, cell differentiation, proliferation, metabolism, and apoptosis. Aberrant miRNA expression is associated with many diseases, notably cancer, cardiovascular disorders, and neurodegenerative conditions. miRNAs are detectable in bodily fluids and can serve as biomarkers; they are also explored as therapeutics, using miRNA mimics to restore lost activity or inhibitors (antagomirs) to suppress harmful miRNAs. Challenges include specific delivery, off-target effects, and stability. Databases such as miRBase catalog known miRNAs, reflecting ongoing discovery and annotation.