武大分子生物学课件CHAPTER_13.pptVIP

Molecular Biology of the Gene, 5/E --- Watson et al. (2004) Part I: Chemistry and Genetics Part II: Maintenance of the Genome Part III: Expression of the Genome Part IV: Regulation Part V: Methods Ch 12 : Mechanisms of Transcription Ch 13 : RNA Splicing Ch 14 : Translation Ch 15 : The Genetic code CHAPTER 13 RNA Splicing Most of the eukaryotic genes are mosaic (嵌合体), consisting of intervening sequences separating the coding sequence Exons (外显子): the coding sequences Introns (内含子) : the intervening sequences RNA splicing: the process by which introns are removed from the pre-mRNA. Alternative splicing (可变剪接): some pre-mRNAs can be spliced in more than one way , generating alternative mRNAs. 60% of the human genes are spliced in this manner. Topic 1 : THE CHEMISTRY OF RNA SPLICING Sequences within the RNA Determine Where Splicing Occurs The borders between introns and exons are marked by specific nucleotide sequences within the pre-mRNAs. The intron is removed in a Form Called a Lariat (套马索) as the Flanking Exons are joined Two successive transesterification: Step 1: The OH of the conserved A at the branch site attacks the phosphoryl group of the conserved G in the 5’ splice site. As a result, the 5’ exon is released and the 5’-end of the intron forms a three-way junction structure. Step 2: The OH of the 5’ exon attacks the phosphoryl group at the 3’ splice site. As a consequence, the 5’ and 3’ exons are joined and the intron is liberated in the shape of a lariat. Exons from different RNA molecules can be fused by Trans-splicing Trans-splicing: the process in which two exons carried on different RNA molecules can be spliced together. Topic 2 THE SPLICESOME MACHINERY RNA splicing is carried out by a large complex called spliceosome The above described splicing of introns from pre-mRNA are mediated by the spliceosome. The spliceosome comprises about 150 proteins and 5 snRNAs (?). Many functions of the spliceosome are carried out by its RNA components. The five R