Small RNA are <200 nt (nucleotide) in length, and are usually non-coding RNA molecules. RNA silencing is often a function of these molecules, with the most common and well-studied example being RNA interference (RNAi), in which endogenously expressed microRNA (miRNA) or exogenously derived small interfering RNA (siRNA) induces the degradation of complementary messenger RNA. Other classes of small RNA have been identified, including piwi-interacting RNA (piRNA) and its subspecies repeat associated small interfering RNA (rasiRNA). Small RNA "is unable to induce RNAi alone, and to accomplish the task it must form the core of the RNA–protein complex termed the RNA-induced silencing complex (RISC), specifically with Argonaute protein".
Small RNA have been detected or sequenced using a range of techniques, including directly by MicroRNA sequencing on several sequencing platforms, or indirectly through genome sequencing and analysis. Identification of miRNAs has been evaluated in detecting human disease, such as breast cancer. Evaluating small RNA is useful for certain kinds of study because its molecules "do not need to be fragmented prior to library preparation".
Kinds of small RNA include:
- microRNA (miRNA)
- Piwi-interacting RNA (piRNA)
- small interfering RNA (siRNA)
- small nucleolar RNA (snoRNA)
- tRNA-derived small RNA (tsRNA)
- small rDNA-derived RNA (srRNA)
- small nuclear RNA, also commonly referred to as U-RNA
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