分子生物学（张海红）CHAPTER 13.pptVIP

Chapter 13RNA 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. RNA splicing Alternative splicing RNA editing mRNA transport RNA splicing Chemistry of RNA splicing The splicesome machinery Splicing pathways 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 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. 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. Trans-splicing: the process in which two exons carried on different RNA molecules can be spliced together. RNA splicing Chemistry of RNA splicing The splicesome machinery Splicing pathways 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 RNAs (U1, U2, U4, U5, and U6, 100-300 nt) are called small nuclear RNAs (snRNAs). The complexes of snRNA and proteins are called small nuclear ribonuclear proteins (snRNP) The spliceosome is the largest snRNP, and the exact makeup differs at different stages of the splicing reaction Recognizing the 5’ splice site and the branch site. 2. Bringing those sites together. 3. Catalyzing (or helping