microARN

microARN is a small non-coding RNA molecule that functions in RNA silencing and post-transcriptional regulation of gene expression. They are typically about 21 to 24 nucleotides long and originate from longer hairpin precursor transcripts. They are encoded by the genome and are involved in fine-tuning the expression of many protein-coding genes.

Biogenesis: Pol II transcribes pri-miRNA; Drosha-DGCR8 processing to pre-miRNA in nucleus; Exportin-5 transports to cytoplasm; Dicer processes pre-miRNA into a double-stranded miRNA duplex; one strand becomes the mature miRNA and loads into the RNA-induced silencing complex with Argonaute; the miRNA-RISC recognizes target mRNAs by base pairing, especially the seed region; leads to translational repression or mRNA degradation. In plants, near-perfect complementarity can trigger mRNA cleavage; in animals, mostly repression with deadenylation.

Function: regulate development, cell proliferation, differentiation, metabolism, and immune responses; often expressed in a tissue-specific manner and conserved across species; collectively form complex regulatory networks.

Clinical relevance and research: profiling of miRNA expression is used for biomarker discovery; dysregulation is linked to cancers and other diseases; therapeutic strategies include miRNA mimics to replace lost miRNAs and antagonists (antagomirs, LNA inhibitors) to block oncogenic miRNAs; challenges include delivery, specificity, and off-target effects.

History and notes: the first miRNAs identified in Caenorhabditis elegans were lin-4 and let-7 in the 1990s, establishing the miRNA pathway as a conserved mode of gene regulation; term microRNA is widely used in English, with microARN used in Spanish and some other languages.