Fig. 3.1 provides a simplified outline of one route by which a primary transcript can be processed to form a molecule of siRNA. The transcript does not code for an amino acid sequence. In step 2 in Fig. 3.1, the single-stranded primary transcript folds back and creates a region where dsRNA is present. Explain how the double-stranded region of RNA can be held together in step 2 and kept in this state in steps 3, 4 and 5.
Once step 3 is complete, the shorter dsRNA formed by Drosha is moved to the cytoplasm, where it is cut again by Dicer to produce ds siRNA. Suggest why two separate enzymes, Drosha and Dicer, are needed to shorten dsRNA into shorter lengths.
From 2018, siRNA has been used as a therapeutic drug to treat a number of diseases. When siRNA molecules are present in the cytoplasm, they can cause messenger RNA (mRNA) molecules that code for a protein involved in the disease to be cleaved. This stops the protein being synthesised. Describe the differences between a molecule of mRNA and a molecule of siRNA, such as the siRNA shown in step 5 in Fig. 3.1.
Referring to Fig. 3.2, state why the passenger strand must be detached and released from the RISC.
The purpose of siRNA therapy is to stop or reduce the synthesis of a protein involved in the disease being treated. A target mRNA molecule can be cleaved at another site by a RISC carrying a different siRNA. Suggest how cutting mRNA at different positions will affect protein synthesis in different ways and explain how these different effects can lead to no functioning protein.