sncRNAs

Small non-coding RNAs (sncRNAs) are a diverse class of RNA molecules that do not encode proteins but regulate gene expression and other cellular processes. They are typically short, ranging from about 20 to a few hundred nucleotides in length, and include microRNAs (miRNAs), small interfering RNAs (siRNAs), piwi-interacting RNAs (piRNAs), small nucleolar RNAs (snoRNAs), and small nuclear RNAs (snRNAs).

miRNAs and siRNAs are usually 21–24 nucleotides long and operate through RNA interference pathways. miRNAs are transcribed as primary transcripts by RNA polymerase II, processed by Drosha to precursor miRNAs, exported to the cytoplasm, and cleaved by Dicer to mature miRNA duplexes; one strand guides Argonaute in the RISC to repress target mRNAs. siRNAs arise from long double-stranded RNAs and are processed by Dicer into single-stranded guides that direct RISC to cleave complementary transcripts. piRNAs are 26–31 nt and are generated independently of Dicer; they associate with PIWI proteins and primarily suppress transposable elements in the germline, often through transcriptional silencing and methylation. snoRNAs, typically 60–300 nt, guide chemical modifications of ribosomal RNA and other RNAs via C/D-box and H/ACA-box families. snRNAs, about 100–200 nt, are core components of the spliceosome and participate in pre-mRNA splicing as part of snRNPs.

sncRNAs have roles in development, immunity, and disease. They serve as biomarkers and as targets or tools in therapeutics, with challenges in delivery, specificity, and off-target effects. Advances in sequencing and RNA delivery technologies continue to expand their study and clinical potential.