## The 5' Cap Structure in Eukaryotic mRNA **Key Point:** The 5' cap (7-methylguanosine cap) is added co-transcriptionally to eukaryotic mRNA and serves multiple critical functions. ### Primary Functions of the 5' Cap 1. **Protection from degradation** - Shields mRNA from 5' → 3' exonuclease activity - Significantly increases mRNA half-life in the cytoplasm - Essential for mRNA stability 2. **Translation enhancement** - Recognized by cap-binding proteins (eIF4E in the eIF4F complex) - Facilitates ribosome recruitment and scanning to the start codon - Increases translation efficiency by 5–10 fold 3. **Nuclear export** - Required for efficient transport through nuclear pore complexes - Recognized by export machinery ### Why Other Options Are Incorrect | Function | Reality | |----------|----------| | Splicing recruitment | snRNPs are recruited to splice sites (GU-AG rule), not the 5' cap | | Direct ribosome recruitment | Ribosome scanning occurs after cap recognition; the cap does not directly bind ribosomes | | Transcription termination prevention | Termination is controlled by polyadenylation signals and termination factors, not the 5' cap | **High-Yield:** The 5' cap is added **within seconds** of transcription initiation (after ~20–30 nucleotides are synthesized), making it a co-transcriptional modification. **Mnemonic:** **CAP-IT** = Cap Addition Protects, Initiates Translation, and Transports [cite:Lehninger Principles of Biochemistry 7e Ch 26]
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