## Pre-mRNA Splicing and the Spliceosome **Key Point:** The spliceosome is a large ribonucleoprotein complex composed of **small nuclear RNAs (snRNAs)** and associated proteins (snRNPs). The snRNPs themselves catalyze the transesterification reactions that remove introns. ### Spliceosome Components and Mechanism | Component | Role | Function | |-----------|------|----------| | **U1 snRNP** | Recognizes 5' splice site | Binds GU sequence | | **U2 snRNP** | Recognizes branch point | Binds UACUAAC sequence | | **U4/U6 snRNP** | Catalytic core | Positions substrates | | **U5 snRNP** | Exon alignment | Bridges exons | | **Proteins (Sm, non-Sm)** | Structural support | Assembly and stability | **High-Yield:** The spliceosome is a **self-splicing ribozyme** — the catalytic activity resides in the **snRNA** (specifically U6 and U2 snRNAs), not in protein enzymes. This is why it is classified as a ribozyme. **Mnemonic:** **"U1-U2-U4-U6-U5"** — Remember the five major snRNPs. **U3 is for rRNA processing** (not pre-mRNA splicing). ### Splicing Mechanism (Two Transesterification Steps) ```mermaid flowchart TD A["Pre-mRNA with intron"]:::outcome --> B["Step 1: 2'-OH of branch point A attacks 5' splice site"]:::action B --> C["Free 3'-OH exon 1 + lariat-shaped intermediate"]:::outcome C --> D["Step 2: 3'-OH of exon 1 attacks 5' end of exon 2"]:::action D --> E["Joined exons + excised intron lariat"]:::outcome ``` **Clinical Pearl:** Mutations in splice sites (especially the conserved GU at the 5' end and AG at the 3' end) cause ~15% of human genetic diseases by disrupting normal splicing, leading to exon skipping or intron retention.
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