The 5' Cap
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A eukaryotic pre-mRNA's 5' end is capped with a modified guanine shortly after transcription begins, protecting the RNA from degradation.
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The 5' cap is added while RNA polymerase II is still transcribing the gene, not after transcription finishes.
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The 5' cap is added at the RNA's 5' end specifically — the corresponding 3' end is protected by a separate modification.