Biochemistry 2280A Lecture Notes - Lecture 19: Small Nuclear Rna, Precursor Mrna, Branch Point

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In eukaryotic cells, dna is enclosed within the nucleus. Transcription takes place in the nucleus, but protein synthesis takes place on ribosomes in the cytoplasm. The enzymes responsible for rna processing ride on the phosphorylated tail of eukaryotic rna polymerase ii as it synthesizes an rna molecule, and they process the transcript as it emerges from the polymerase. Capping of the 5" end of the message with 7-methylguanosine. Occurs after rna polymerase ii synthesizes ~25 nucleotides of rna, long before it has completed transcribing the whole gene. Marks the 5" end of mrna as being intact: make sure that both ends of the mrna are present & message is complete before protein synthesis. Required for: mrna export from nucleus no cap = no export, translation of mrna. 3" polyadenylation addition of a long a tract to the 3" end of mrna. Almost all eukaryotic mrnas end with a long (~300 base) polya tail.

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