miRNA

miRNA stands for microRNA, a class of endogenous, small non-coding RNAs about 21 to 24 nucleotides long that regulate gene expression at the post-transcriptional level in eukaryotes. miRNAs typically originate from longer primary transcripts that fold into hairpin structures and are conserved across species. They are encoded by miRNA genes that can reside in intergenic regions or within introns of protein-coding genes. The first miRNAs were discovered in Caenorhabditis elegans, and the pathway is conserved in plants and animals.

Biogenesis: Pri-miRNA transcripts are processed in the nucleus by the Drosha-DGCR8 microprocessor into precursor miRNA (pre-miRNA) hairpins of about 70 nucleotides. Pre-miRNA is exported to the cytoplasm by Exportin-5. In the cytoplasm, Dicer cleaves the pre-miRNA to a short duplex containing the mature miRNA strand and a passenger strand. The duplex is loaded into an Argonaute-containing RNA-induced silencing complex (RISC); the guide strand is retained while the passenger strand is degraded.

Mechanism: The mature miRNA directs RISC to target mRNAs by base pairing, typically with imperfect complementarity in animals. The seed region, nucleotides 2–7, is crucial for target recognition. Binding usually results in translational repression and/or mRNA deadenylation and decay. In plants, miRNAs often have near-perfect complementarity, leading to direct mRNA cleavage.

Role and applications: miRNAs regulate development, differentiation, metabolism, and immune function. Dysregulation is associated with diseases such as cancer, cardiovascular and neurological disorders. miRNA expression profiles can serve as diagnostic or prognostic biomarkers, and miRNA-based therapies are being explored, including mimics and inhibitors. Tools for detection include qRT-PCR, microarrays, and small RNA sequencing.