SiRNA, or small interfering ribonucleic acid, is a type of RNA that is involved in a number of biological processes, most notably RNA interference. RNA interference is a regulatory process that is used to control and limit the expression of some genes. While most RNA is single stranded, siRNA is made up of two complementary strands of nucleotides similar to those in DNA, the primary biological unit for the storage of genetic information. One strand of the double-stranded RNA may bind to messenger RNA in order to inhibit its role in protein synthesis, thereby limiting the expression of a particular gene. Small interfering RNA is sometimes alternatively referred to as silencing or short interfering RNA.
SiRNA is primarily involved in the regulatory process of RNA interference. Long, double-stranded RNA strands are cut apart into much shorter strands by an enzyme called dicer. The two strands then separate; one degrades while the other is integrated into an RNA-induced silencing complex made up of a selection of different proteins and the single siRNA strand, known as the guide strand. The guide strand is used to recognize the specific RNA strand with which the complex is intended to interfere, or "silence." The protein complex cleaves the targeted RNA strand, thereby preventing it from being translated into a protein.
RNA interference guided by siRNA is an important tool used in many different biology laboratories because of its ability to target and suppress the expression of specific genes. Researchers do, however, face many problems while implementing RNA interference as an effective research method. The short siRNA strands sometimes bind incompletely to the wrong RNA sequences, so nonspecific binding is sometimes a problem. Additionally, many different viruses act through double stranded RNA, so an organism's immune system may recognize siRNA as a threat and mount an immune response against it. Regardless of these problems, researchers have been able to use RNA interference to substantially reduce the expression of a given gene in order to determine the exact function of the gene in question.
The immune systems of various organisms, including plants, mammals, and insects, actually rely on RNA interference to some extent. This is particularly true when the organism needs to combat a viral infection. The RNA-induced silencing complex can target and deactivate viral RNA to prevent it from damaging the host organism. While the roles of siRNA and RNA interference are well understood in plants and some other organisms, the exact level of involvement of RNA in the mammalian immune system remains poorly understood.