Figure 1

How RNAi suppresses genes: Short siRNA pieces unwind into single strand RNAs, which then combine with proteins to form RISC. The RISC then binds with a native mRNA molecule that complements the short siRNA sequence. If the pairing of native mRNA and siRNA piece is essentially perfect, the native mRNA is cut into useless RNA fragments that can't be translated. However, if the nucleotide base pairing is less than perfect, then the RISC complex binds to the mRNA and prevents the ribosome movement along the mRNA, also halting the translation, net resulting in no protein tranlation. (http://www.ambion.com/techlib/tn/101/7.html, Last Accessed: 9th April, 2010)