what is RNA silencing and how is it achieved RNAi and miRNA?

Small interfering RNA (siRNA), sometimes known as short interfering RNA or silencing RNA, are a class of 20-25 nucleotide-long double-stranded RNA molecules that play a variety of roles in biology. Most notably, siRNA is involved in the RNA interference (RNAi) pathway where the siRNA interferes with the expression of a specific gene. In addition to their role in the RNAi pathway, siRNAs also act in RNAi-related pathways, e.g. as an antiviral mechanism or in shaping the chromatin structure of a genome; the complexity of these pathways is only now being elucidated. These siRNAs are derived from processing of the trigger dsRNA by an enzyme known as Dicer in an RNase III-like fashion. The mechanism involves the recruitment of siRNA duplex products into a multi-protein siRNA complex, known as the RISC (RNA-Induced Silencing Complex). The RISC is then thought to be guided to the target mRNA of interest where the siRNA duplex interacts in a sequence-specific manner to mediate catalytic cleavage.

MicroRNAs (miRNA) are single-stranded RNA molecules of about 21-23 nucleotides in length thought to regulate the expression of other genes. miRNAs are encoded by genes that are transcribed from DNA but not translated into protein (non-coding RNA); instead they are processed from primary transcripts known as pri-miRNA to short stem-loop structures called pre-miRNA and finally to functional miRNA. Mature miRNA molecules are partially complementary to one or more messenger RNA (mRNA) molecules, and they function by targeting mRNAs for translational repression, degradation, or both. These molecules are emerging as important modulators in cellular pathways such as growth and proliferation, apoptosis, and developmental timing.

MicroRNAs were first described in 1993 by Lee, et al. (Lee RC, Feinbaum RL, Ambros V (1993) The C. elegans heterochronic gene lin-4 encodes small RNAs with antisense complementarity to lin-14. Cell 75: 843-854.) To date, thousands of miRNAs have been identified in organisms from viruses to primates through cloning and sequencing, or computational prediction based on strong conservation of miRNA sequence motifs. miRNA data are managed and annotated by the miRBase Sequences Database, hosted by the Sanger Institute as a publicly accessible repository.

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