Small interfering RNA (siRNA) is short, double-stranded RNA, about 21 to 23 nucleotides long, that silences a specific gene by causing its mRNA to be destroyed before it can be translated. This is called RNA interference (RNAi). It controls gene expression after transcription.
How siRNA silences a gene
1. A long molecule of double-stranded RNA is present in the cell. It may come from a virus or be introduced by scientists.
2. An enzyme called Dicer cuts the long double-stranded RNA into siRNA molecules.
3. The siRNA is loaded into a protein complex called RISC (RNA-induced silencing complex). The two strands separate and one strand is kept.
4. The single siRNA strand guides RISC to an mRNA molecule with a complementary base sequence. The siRNA binds to it by complementary base pairing.
5. An enzyme in RISC cuts the mRNA, which is then broken down.
6. The mRNA cannot be translated, so no protein is made from that gene.Because the siRNA must be exactly complementary to its target, each siRNA silences only one gene. RISC is not used up, so one complex can destroy many mRNA molecules.Uses of siRNA
• Research: scientists make siRNA complementary to a gene and add it to cells. By silencing the gene and seeing what changes, they can work out what the gene does.
• Medicine: siRNA can be designed to silence a harmful gene, such as one producing a faulty protein or a gene of a virus. Some siRNA drugs are already approved for treating rare genetic diseases.
• Defence: in plants and some animals RNAi helps to protect cells against viruses.
How siRNA silences a gene1. A long molecule of double-stranded RNA is present in the cell. It may come from a virus or be introduced by scientists.
2. An enzyme called Dicer cuts the long double-stranded RNA into siRNA molecules.
3. The siRNA is loaded into a protein complex called RISC (RNA-induced silencing complex). The two strands separate and one strand is kept.
4. The single siRNA strand guides RISC to an mRNA molecule with a complementary base sequence. The siRNA binds to it by complementary base pairing.
5. An enzyme in RISC cuts the mRNA, which is then broken down.
6. The mRNA cannot be translated, so no protein is made from that gene.Because the siRNA must be exactly complementary to its target, each siRNA silences only one gene. RISC is not used up, so one complex can destroy many mRNA molecules.Uses of siRNA
• Research: scientists make siRNA complementary to a gene and add it to cells. By silencing the gene and seeing what changes, they can work out what the gene does.
• Medicine: siRNA can be designed to silence a harmful gene, such as one producing a faulty protein or a gene of a virus. Some siRNA drugs are already approved for treating rare genetic diseases.
• Defence: in plants and some animals RNAi helps to protect cells against viruses.