MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression by interacting with target mRNAs. They are primarily transcribed from DNA into primary miRNAs (pri-miRNAs), which are processed into precursor miRNAs (pre-miRNAs) and mature miRNAs. miRNAs typically bind to the 3' untranslated region (3' UTR) of target mRNAs to induce degradation and translational repression, but can also interact with other regions such as the 5' UTR, coding sequence, and gene promoters. Under certain conditions, miRNAs can activate translation or regulate transcription. The interaction of miRNAs with their targets is dynamic and influenced by factors such as subcellular location, abundance of miRNAs and target mRNAs, and the affinity of miRNA-mRNA interactions. miRNAs can be secreted into extracellular fluids and transported to target cells via vesicles like exosomes or by binding to proteins such as Argonautes. Extracellular miRNAs function as chemical messengers to mediate cell-cell communication. miRNA biogenesis occurs through canonical and non-canonical pathways. The canonical pathway involves the processing of pri-miRNAs by the microprocessor complex (Drosha and DGCR8) into pre-miRNAs, which are then exported to the cytoplasm and processed by Dicer into mature miRNAs. Non-canonical pathways include Drosha/DGCR8-independent and Dicer-independent pathways, where miRNAs are processed from introns or other sources without the need for Drosha or Dicer. miRNAs regulate gene expression through miRISC complexes, which can induce translational repression, mRNA deadenylation, and decapping. miRNAs can also activate translation under certain conditions. miRISC can localize in various subcellular compartments, including the nucleus, cytoplasm, endosomes, and mitochondria, where it can regulate transcription, splicing, and chromatin structure. miRNAs can also be secreted into extracellular fluids and transported to target cells, where they can act as biomarkers or signaling molecules for intercellular communication. Recent studies have shown that miRNAs are dynamic and can regulate gene expression in response to cellular conditions. The localization and activity of miRNAs are influenced by factors such as subcellular compartmentalization, miRNA/mRNA abundance, and the availability of miRISC components. miRNAs play important roles in various physiological and pathological processes, and their dysregulation is associated with many human diseases. The study of miRNA biogenesis, mechanisms of action, and circulation is an active area of research with implications for understanding gene regulation and developing therapeutic strategies.MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression by interacting with target mRNAs. They are primarily transcribed from DNA into primary miRNAs (pri-miRNAs), which are processed into precursor miRNAs (pre-miRNAs) and mature miRNAs. miRNAs typically bind to the 3' untranslated region (3' UTR) of target mRNAs to induce degradation and translational repression, but can also interact with other regions such as the 5' UTR, coding sequence, and gene promoters. Under certain conditions, miRNAs can activate translation or regulate transcription. The interaction of miRNAs with their targets is dynamic and influenced by factors such as subcellular location, abundance of miRNAs and target mRNAs, and the affinity of miRNA-mRNA interactions. miRNAs can be secreted into extracellular fluids and transported to target cells via vesicles like exosomes or by binding to proteins such as Argonautes. Extracellular miRNAs function as chemical messengers to mediate cell-cell communication. miRNA biogenesis occurs through canonical and non-canonical pathways. The canonical pathway involves the processing of pri-miRNAs by the microprocessor complex (Drosha and DGCR8) into pre-miRNAs, which are then exported to the cytoplasm and processed by Dicer into mature miRNAs. Non-canonical pathways include Drosha/DGCR8-independent and Dicer-independent pathways, where miRNAs are processed from introns or other sources without the need for Drosha or Dicer. miRNAs regulate gene expression through miRISC complexes, which can induce translational repression, mRNA deadenylation, and decapping. miRNAs can also activate translation under certain conditions. miRISC can localize in various subcellular compartments, including the nucleus, cytoplasm, endosomes, and mitochondria, where it can regulate transcription, splicing, and chromatin structure. miRNAs can also be secreted into extracellular fluids and transported to target cells, where they can act as biomarkers or signaling molecules for intercellular communication. Recent studies have shown that miRNAs are dynamic and can regulate gene expression in response to cellular conditions. The localization and activity of miRNAs are influenced by factors such as subcellular compartmentalization, miRNA/mRNA abundance, and the availability of miRISC components. miRNAs play important roles in various physiological and pathological processes, and their dysregulation is associated with many human diseases. The study of miRNA biogenesis, mechanisms of action, and circulation is an active area of research with implications for understanding gene regulation and developing therapeutic strategies.