Small interfering RNA

From Freepedia

Small interfering RNA (siRNA), sometimes known as short interfering RNA, are a class of 20-25 nucleotide-long RNA molecules that interfere with the expression of genes. They are naturally produced as part of the RNA interference (RNAi) pathway by the enzyme Dicer. They can also be exogenously (artificially) introduced by investigators to bring about the knockdown of a particular gene. SiRNA's have a well defined structure. Briefly, this is a short (usually 21-nt) double-strand of RNA (dsRNA) with 2-nt overhangs on either end, including a 5' phosphate group and a 3' hydroxy (-OH) group.

Since transfection of an exogenous siRNA is only transient, and the dsRNA structure cannot easily be permanently introduced. This can be achieved by the introduction of a loop between the two strands, so that a single transcript can be processed into a functional siRNA. This transcription cassette usually uses an RNA polymerase III promoter, which direct the transcription of small nuclear RNA's, such as U6 or H1. It is assumed (although not known for certain) that the resulting short hairpin RNA (shRNA) transcript is processed by Dicer.

Introduction of too much siRNA can result in the non-specific knockdown of genes due to activation of the interferon pathway. This is probably activated by the dsRNA sensor PKR.

External links

• First description of siRNA's (1999).
• siDirect: a web-based online software system for computing siRNA sequences
• Paper describing siDirect
• Paper describing effectivity of siDirect
• HuSiDa: Human siRNA Database
• siRNAdb: a database of siRNA sequences