lecture20 - Splicing Terminology Exon Intron Exon DNA...

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Splicing Terminology Primary Transcript Mature RNA DNA Intron Exon Exon 3’ Splice Site 5’ Splice Site + intron
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Two Types of Primary Transcripts 1) Simple Transcripts --produce 1 mature mRNA and 1 type of polypeptide 2) Complex Transcripts --produce 2 or more different mRNAs and polypeptides --have >1 site for cleavage and polyadenylation or --have alternative splicing patterns
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Complex Transcript Production Poly A Site 5’ Splice Site 3’ Splice Sites Cap Poly A Sites DNA Primary Transcripts Cap 1 2 Cleavage at Site 1 Cleavage at Site 2 (A)n (A)n (A)n Cleavage and Polyadenylation (A)n (A)n Splicing = cap
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Splicing of mRNA Primary Transcripts catalyzed by large RNA/protein complex = spliceosome --3X10 6 daltons --45 proteins --5 RNAs = small nuclear RNAs (snRNAs) --U1, U2, U4, U5 and U6 --snRNAs are complexed to proteins = ribonucleoproteins = snRNPs (“snurps”) involves removal of introns by 2 transesterification rxns --between 5’ splice site and branch-point A --between 5’ exon and 3’ splice site excised intron released as lariat molecule
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Intron Removal Spliceosome positions A at branch site near 5’ splice site 2’ OH group of A attacks 5’ splice site --moves ester bond from intron/exon boundary to lariat -- forms a 2’,5’ phospodiester bond New phosphodiester bond forms between exons New 3’ OH of 5’ exon attacks 3’ splice site
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Splicing U1 snRNA of U1 snRNP binds to complementary sequnce at 5’ end of intron
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lecture20 - Splicing Terminology Exon Intron Exon DNA...

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