# Explain how small interfering RNA (siRNA) molecules silence gene expression in eukaryotic cells.

> IB DP Biology Higher Level (2023 syllabus) — D2.2 Gene expression (HL only) · Explain · 4 marks

## Mark scheme (4 marks)

1. Double-stranded RNA (dsRNA) is cleaved/processed by the enzyme Dicer into short siRNA duplexes (approximately 21–23 nucleotides in length).
2. The siRNA duplex is incorporated into the RNA-induced silencing complex (RISC), and the passenger strand is discarded, leaving the single-stranded guide strand.
3. The guide strand within RISC base-pairs with complementary sequences on the target mRNA by complementary base pairing.
4. The Argonaute protein (Slicer/AGO2) within RISC cleaves/degrades the target mRNA, preventing its translation and thereby silencing gene expression.

## Key terms

- [siRNA](https://www.gradenine.co.uk/glossary/sirna)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-small-interfering-rna-sirna-4641a8e2) · Published by Druglandscape Ltd.