RNA的剪接 RNA Splicing.pptVIP

Figure 13-19 The AT-AC spliceosome U11 and U12 are in places of U1 and U2, respectively EXON SHUFFLING Topic 5:外显子重排 所有的真核基因都有内含子，而原核基因却很少有内含子. 有两个假说 : 1. Introns early model – introns existed in all organisms but have been lost from bacteria. 2. Intron late model – introns never existed in bacteria but rather arose later in evolution. 外显子的重排可以产生新基因，从而编码新蛋白 为什么真核生物要保留内含子? 内含子去除的需要，可以通过选择性剪接由单基因产生多蛋白. 2. 内含子把编码序列分成了外显子，可以通过外显子重排产生新基因(new genes). 1. The borders between exons and introns within a gene often coincide with the boundaries between domains within the protein encoded by that gene. 有些实验证实外显子重排的存在: For example: DNA-binding protein Figure 13-21 2. Many genes, and proteins they encode, have apparently arisen during evolution in part via exon duplication and divergence. 3. Related exons are sometimes found in unrelated genes. Exons have been reused in genes encoding different proteins Figure 13-22 Topic 6RNA EDITING自学 CHAPTER 13 RNA Splicing RNA editing is another way of changing the sequence of an mRNA I. Site specific deamination : 1. A specifically targeted C residue within mRNA is converted into U by the deaminase. 2. The process occurs only in certain tissues or cell types and in a regulated manner. RNA editing Figure 13-25 The human apolipoprotein gene Stop code In liver In intestines Figure 13-25 3. Adenosine deamination also occurs in cells. The enzyme ADAR (adenosine deaminase acting on RNA) convert A into Inosine. Insone can base-pair with C, and this change can alter the sequence of the protein. 4. An ion channel expressed in mammalian brains is the target of Adenosine deamination. II Guide RNA-directed uridine insertion or deletion. 1. This form of RNA editing is found in the mitochondria of trypanosomes. 2. Multiple Us are inserted into specific region of mRNAs after transcription (or US may be deleted). 3. The addition of Us to the message changes codons and reading frames, completely altering the “meaning” of the message. 4. Us are i