## Most Common Site of Splicing Errors **Key Point:** The 5' splice site (consensus sequence GU at the intron start) is the most frequently mutated and error-prone region in pre-mRNA splicing, accounting for approximately 15% of all splicing defects in genetic diseases. ### Why the 5' Splice Site is Most Vulnerable **High-Yield:** The 5' splice site has the least stringent consensus sequence among all splicing signals, making it highly susceptible to point mutations that disrupt the GU dinucleotide or flanking nucleotides. Even single nucleotide substitutions (e.g., G→A at position +1) can abolish recognition by the U1 snRNP component of the spliceosome. ### Comparison of Splicing Signal Sequences | Signal Element | Consensus Sequence | Mutation Frequency | Clinical Impact | |---|---|---|---| | 5' splice site | GU (intron start) | **Highest (~15%)** | Exon skipping, intron retention | | 3' splice site | AG (intron end) | Moderate (~10%) | Cryptic splice site activation | | Branch point | YNYURAY | Lower (~5%) | Weak splicing, exon skipping | | Polypyrimidine tract | Pyrimidine-rich | Lowest (~2%) | Rarely rate-limiting | ### Mechanism of 5' Splice Site Recognition 1. U1 snRNP binds to the 5' GU sequence via base pairing 2. The GU dinucleotide is absolutely conserved across species 3. Mutations at positions +1 (G) or +2 (U) are most disruptive 4. Flanking nucleotides (positions -3 to +6) also contribute to binding strength **Clinical Pearl:** β-thalassemia and spinal muscular atrophy (SMA) frequently result from 5' splice site mutations, demonstrating the clinical prevalence of this defect. **Mnemonic:** **GU-FIRST** — the 5' **G**-**U** is the **F**irst and most **I**mportant **R**ecognition **S**ite, **T**hus most prone to error.
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