Press the start button then select the most appropriate word to fill each gap from the drop-down list. Each word in the list is used only once. Hint: read ahead and think about grammar to narrow down the choice of possible answers. When you think that you have it right, press the check sequence button. There is a 5 second penalty if you are wrong!
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The abbreviation siRNA stands for small RNA. These are short (20-25bp) regions of -stranded RNA that can be used to inhibit the expression of particular with the cell, by targeting the relevant sequences for destruction by .
The system exploits an important cellular control mechanism called RNA interference that is also involved in defence against infection. Unlike normal cellular processes, viral and viral gene expression can lead to the formation of long double stranded molecules. These are recognised by a cellular protein called . This protein digests the long double stranded RNA to give short double stranded RNA sequences that are mimicked using siRNA. The short sequences are incorporated into a second protein complex called (RNA induced silencing complex), that the siRNA and uses one of the strands to seek out any mRNA sequences that carry a sequence.
When the RISC complex finds a matching sequence a protein component called cleaves the mRNA by hydrolysis of the bonds at a specific location within the matching sequence. The two that result from this are then degraded by that digest the rest of the mRNA starting from the newly exposed 3' and 5' ends. This targeted destruction of viral mRNA sequences inhibits viral replication.
The system can be exploited as a valuable tool, allowing the of genes in cells to be explored by inhibiting their expression using siRNA sequences matching the gene of interest. The siRNA can be synthesised and added to cells in . Alternatively, it can be produced within the cells by transfecting them with that expresses an RNA sequence which is then processed to give siRNA.