Explain how small interfering RNA (siRNA) molecules silence gene expression in eukaryotic cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
siRNA molecules are short double‑stranded RNAs that are produced when the enzyme Dicer cleaves longer double‑stranded RNA into 21–23‑nt duplexes. The duplex is loaded into the RNA‑induced silencing complex (RISC); the passenger strand is removed, leaving the single‑stranded guide strand. This guide strand base‑pairs with complementary sequences on the target mRNA. The Argonaute protein (AGO2) in RISC then cleaves the mRNA, leading to its degradation and preventing translation, thereby silencing gene expression.
Examiner tips
- Mention Dicer processing and siRNA length; specify RISC loading and passenger strand removal; describe guide strand base‑pairing; state Argonaute cleavage and mRNA degradation. Keep each point concise to match the 4 marks.
Common mistakes
- Confusing siRNA with miRNA; omitting the passenger strand removal step; failing to name Argonaute (AGO2) or the cleavage mechanism; using vague terms like "silencing" without explaining mRNA degradation.
Mark scheme (4 marks)
- Double-stranded RNA (dsRNA) is cleaved/processed by the enzyme Dicer into short siRNA duplexes (approximately 21–23 nucleotides in length).
- The siRNA duplex is incorporated into the RNA-induced silencing complex (RISC), and the passenger strand is discarded, leaving the single-stranded guide strand.
- The guide strand within RISC base-pairs with complementary sequences on the target mRNA by complementary base pairing.
- The Argonaute protein (Slicer/AGO2) within RISC cleaves/degrades the target mRNA, preventing its translation and thereby silencing gene expression.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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