U4 leaves the complex, allowing the RNa components of U2 and U6 to base pair produce the active site. The branch site A attacks the 5 splice site, forming the 3-way junction and C complex. The 5 splice site then attacks the 3 splice site, freeing the intron lariat and forming the mRNA product. Group Intron splicing (2): the chemistry is the same as that of the spliceosome-mediated mRNA splicing, but the splicing is catalyzed by the intron RNA itself, which is also named as self-splicing Group I intron splicing Chemistry: the 3 OH of an exogenous G nucleotide or nucleoside attacks the 5 splice ite to undergo the first ester-transfer reaction. Then the 3'OH of the 5 exon attacks 5'phosphate of at the 3 splice site, resulted in the release of a linear intron and gated exons. The splicing is catalyzed by the intron rNa itself as the group Il intron Conclusion 7. What is alternative splicing? What is the biological importance of alternative splicing? How alternative splicing is regulated(Note the involvement of the cis-acting elements and trans-acting factors)?(10 points) Alternative splicing (3): many protein-encoded genes in higher eukaryotes contain multiple introns and can be spliced in alternative ways to generate two or more different mRNAs Biological functions(2): Alternative splicing allows individual genes to prod multiple protein isoforms, thereby playing a central part in expanding prote diversity from a limited number of genes and regulating gene expression in higher eukaryotes(2 ). Alternative splicing also has a largely hidden function in quantitative gene control, by targeting RNAs for nonsense-mediated decay. (1) Regulatory mechanisms(5): alternative splicing is regulated by trans-acting factors(activators or repressors) that recognize an arrangement of positive and/or negative cis-acting sequence elements called exonic(or intronic) splicing enhancers or silencers. An interplay between cis-acting sequences and trans-acting factors modulates the splicing of regulated exons. Activators include members of the Sr protein family and can activate splicing by binding to exon splicing enhancers (eses) using RNA-recognition domain and recruiting the splicing machinery using Rs domain. Repressors are frequently members of the heterogeneous nuclear ribonucleoprotein(hnRNP) family, which bind to exonic/intronic splicing silencers blocking specific splice site INote:有些同学不懂什么是 trans- acting,什么是cis- acting;还有的同学回答成 trans- splicing的机制。 Here is some additional material to explain the cis-acting elements and trans-acting factors. The basic concept for how transcription controlled in bacteria was provided by the classic formulation of the model for control of gene expression by Jacob and Monod in 1961. They distinguished between two types of sequences in DNA: sequences that code for trans-acting products; and cis-acting sequences that function exclusively within the dNa (and also rna elementU4 leaves the complex, allowing the RNA components of U2 and U6 to base pair to produce the active site. The branch site A attacks the 5’ splice site, forming the 3-way junction and C complex. The 5’ splice site then attacks the 3’ splice site, freeing the intron lariat and forming the mRNA product. Group Ⅱintron splicing (2’): the chemistry is the same as that of the spliceosome-mediated mRNA splicing, but the splicing is catalyzed by the intron RNA itself, which is also named as self-splicing. Group Ⅰintron splicing. Chemistry: the 3’ OH of an exogenous G nucleotide or nucleoside attacks the 5’splice site to undergo the first ester-transfer reaction. Then the 3’OH of the 5’ exon attacks 5’ phosphate of at the 3’ splice site, resulted in the release of a linear intron and ligated exons. The splicing is catalyzed by the intron RNA itself as the group II intron does. Conclusion: 7. What is alternative splicing? What is the biological importance of alternative splicing? How alternative splicing is regulated (Note the involvement of the cis-acting elements and trans-acting factors)? (10 points) Alternative splicing (3’): many protein-encoded genes in higher eukaryotes contain multiple introns and can be spliced in alternative ways to generate two or more different mRNAs. Biological functions (2’): Alternative splicing allows individual genes to produce multiple protein isoforms, thereby playing a central part in expanding protein diversity from a limited number of genes and regulating gene expression in higher eukaryotes (2’). Alternative splicing also has a largely hidden function in quantitative gene control, by targeting RNAs for nonsense-mediated decay. (1’) Regulatory mechanisms (5’): alternative splicing is regulated by trans-acting factors (activators or repressors) that recognize an arrangement of positive and/or negative cis-acting sequence elements called exonic (or intronic) splicing enhancers or silencers. An interplay between cis-acting sequences and trans-acting factors modulates the splicing of regulated exons. Activators include members of the SR protein family and can activate splicing by binding to exon splicing enhancers (ESEs) using RNA-recognition domain and recruiting the splicing machinery using RS domain. Repressors are frequently members of the heterogeneous nuclear ribonucleoprotein (hnRNP) family, which bind to exonic/intronic splicing silencers, blocking specific splice site [Note: 有些同学不懂什么是 trans-acting，什么是 cis-acting；还有的同学回答成 trans-splicing 的机制。Here is some additional material to explain the cis-acting elements and trans-acting factors. The basic concept for how transcription is controlled in bacteria was provided by the classic formulation of the model for control of gene expression by Jacob and Monod in 1961. They distinguished between two types of sequences in DNA: sequences that code for trans-acting products; and cis-acting sequences that function exclusively within the DNA (and also RNA element