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Note14 - D Base pairing interactions important for...

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Lecture 16 MCB 161 S. Harmer 2/25/2010 mRNA processing Reading: Watson: pp 404 (Figure 12-20); 406-410; 415-432; 452-453 (you can skip boxes 13-1 and 13-2) Problems: see web site I. Pol II CTD carries capping, splicing and polyadenylation factors for transfer to pre-mRNA II. Capping A. Addition of guanosine (G) to 5'-end via a 5'ppp5' linkage. B. 2'-OH methylation of cap to form7-methyl G (me7G); sometimes, adjacent residues also methylated C. Stabilizes mRNA and acts as binding site for small subunit of ribosome to promote translation initiation. III. Polyadenylation A. Non-specific termination of transcription in usually A/T-rich regions B. Cleavage near AAUAAA, before GT repeat sequence followed by addition of ~50 - 300 A residues. C. Stabilizes mRNA and can aid in translation. IV. Splicing (intron removal) A. Most genes of multicellular eukaryotes interrupted by multiple introns. B. Biochemistry of splicing - transesterification and lariat formation. C. Spliceosome for catalysis of splicing is an assembly of snRNP.
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Unformatted text preview: D. Base pairing interactions important for spliceosome assembly and function. E. Note similarity between splicing mechanisms of pre-mRNAs and the group II self-splicing introns! RNAs in the splicing snRNPs likely evolved from intronic sequences; remnant of the ‘RNA world’ Lecture 16 MCB 161 S. Harmer 2/25/2010 • U1 binds 5’ splice junction (RNA/RNA basepairing) • U2 binds to branch site • U4/U6 (basepaired to each other) and U5 bind • Progressive rearrangements: • U1 and U4 leave (requires helicase) • U5 binds both exons • U6 basepairs with U2 to form the active site • U6/U2 catalyze first transesterification reaction (lariat intermediate formed) • U6/U2 catalyze second transesterification (exons spliced) • U2/U5/U6 leave, with lariat intron • Lariat is degraded, but U2/U5/U6 dissociate and act in another splicing reaction Steps of the spliceosome-mediated splicing reaction: Fig. 13.8...
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