BIOL-L211 Lecture 26 Outline of Last Lecture I. EpigeneticsII. Article III. Epigenetic TheoryIV. RNA SplicingOutline of Current Lecture I. The Splicing ReactionII. The SpliceosomeIII. Alternative SplicingCurrent LectureRNA SplicingI. The Splicing ReactionA. Begins with pre-mRNA with an intron between two exonsB. Reaction steps:1. 2' hydroxyl group of an adenine in the branch site attacks a guanine's phosphoryl group in the 5' splice site, breaking the 5' exon-intron phosphodiester bond2. The 5' end of the intron then binds to the adenine forming a three way junction in the branch site3. The 3' hydroxyl group from the 5' exon then connects to the phosphoryl group of a guanine in the 3' splice site (which connects the two exons)4. The intron is released as a lariat (referring to its looped structure)These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.II. The SpliceosomeA. Components of the Spliceosome:1. Proteins2. snRNPs (pronounced "snirps"): small nuclear ribonuclear proteinsa. Include U1, U2, U4, U5, and U6b. Made up of proteins and small nuclear RNA3. Spliceosome composition changes during splicing processB. Action of snRNPs1. Make RNA-RNA, RNA-protein, & protein-protein interactions2. Are specific (ex: recognize the 5' splice site, branch site, and/or 3'splice site)3. Responsible for bringing 5' splice site and branch site close together4. Have catalytic propertiesC. Splicing pathway1. U1 (snRNP): recognizes the 5' splice site2. BBP: protein that identifies the branch site3. U2AF: protein that recognizes the 3' splice site4. BBP displaced by U2 at branch site5. U2AF is removed6. U1 and U2 communicate with U4, U5, and U6 complex*5' splice site is brought closer to the branch site7. U1 and U4 depart the complex8. U2, U5, and U6 remain*They cleave the 5' splice site, form the lariat, and finish splicing exons togetherIII. Alternative SplicingA. mRNA can be spliced multiple ways, resulting in two or more different mature transcripts1. Result is variation in protein products2. Generally regulatedB. Troponin T Gene1. Has 5 exons in pre-mRNA2. Has two alternative spliced forms (4 exons each)3. Not all exons are retained in splicing4. Alternative exon selection is mutually exclusiveC. Mutually Exclusive Splicing1. Steric hindrance: permits mutually exclusive splicingD. Ways to Splice a Gene1. Cassette exon: Skips an exon (deliberately)2. Exon extension: selects an alternative splice site3. Intron retention (least common alternative splicing mechanism; may be a mistake)4. Alternative exons: Similar to example with Troponin
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