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1、,In prokaryotes, transcription produces a nearly exact mRNA copy of the DNA, and the transcript is immediately translated into protein. In eukaryotes, a series of modifications occur to mRNA during and after transcription.,在原核生物中，转录产生的mRNA几乎是DNA的准确拷贝，并且这一转录产物会立即被转译成蛋白质。在真核生物中，转录时以及转录后会对mRNA进行一系列修饰。,
2、Chapter 5 mRNA Modifications in Eukaryotes 第5章 真核生物mRNA的修饰,5.1 Capping 5.2 Polyadenylation 5.3 Splicing 5.4 mRNA Editing 5.5 Experiments,5.1 加帽 5.2 聚腺苷酸化 5.3 剪接 5.4 mRNA编辑 5.5 实验研究,Chapter 5 mRNA Modifications in Eukaryotes 第5章 真核生物mRNA的修饰,5.1 Capping / 加帽,An mRNA that has been transcribed but is no
3、t yet ready for translation is called a pre-mRNA, or a primary transcript. Capping is the process of adding a derivative of guanine nucleotide to the 5 end of the pre-mRNA.,一条已经转录出来但还没有准备好用于转译的mRNA称为前体mRNA或初级转录本。 加帽是在前体mRNA的5末端加上一个鸟嘌呤核苷酸衍生物的过程。,Structure of the cap / 帽的结构,5-3 phospho- diester bond,5
4、-5 triphosphate bond,RNA polymerase II,Discussed in Chapter 4,Capping takes place quite early.,5-Cap,RNA polymerase II,RNA,DNA,Capping process / 加帽过程,CTD: C-terminal domain,RNA polymerase II,DNA,Methyl transferase,Guanylyl transferase,RNA triphosphatase,Functions of the cap structure 帽结构的功能,1. Helps
5、 prevent degradation 帮助防止降解 2. Helps transport into cytoplasm 帮助转运到细胞质中 3. Enhances translation / 增强转译 4. Helps remove the first intron 帮助去除第一个内含子,1. Helps prevent degradation 帮助防止降解,RNase,RNase,5-5 triphosphate bond,5-3 phospho diester bond,5-3 phospho diester bond,2. Helps transport into cytoplasm
6、 帮助转运到细胞质中,Discussed in Chapter 7,3. Enhances translation / 增强转译,Cap-binding protein,No translation occurs.,RNA polymerase II,RNA,DNA,4. Helps remove the first intron 帮助去除第一个内含子,The first intron,AAAAAAA - - - - - - AAAA,5.2 Polyadenylation / 聚腺苷酸化,Poly(A) tail,Polyadenylation,Pre-mRNA,Polyadenylatio
7、n is a modification process in which a string of about 250 adenine nucleotides is added to the 3 end of the transcript.,Polyadenylation does not occur at the natural end,The polyadenylation signal / 聚腺苷酸化信号,AAAAAAA - - - - - - AAAA,Poly(A) tail,Polyadenylation,Pre-mRNA,Polyadenylation signal has a t
8、ypical typical sequence of AAUAAA. This sequence gives an instruction as “to cut the mRNA about 20 nucleotides downstream, near a GU-rich sequence”.,AAUAAA,GU,The cleavage complex / 切割复合体,Cleavage complex,CPSF (cleavage and polyadenylation specificity factor),CstF (cleavage stimulation factor),CFI (
9、cleavage factor I),CFII (cleavage factors II),Poly(A)-Binding Protein,CTD: C-terminal domain,Proteins for capping at CTD,Proteins for polyadenylation at CTD,Functions of the poly(A) tail poly(A)尾的功能,AAAAAAA - - - - - - AAAA,AAAAAAA - - -,A,A,A,A,A,A,The main function of poly(A) tail is to protect th
10、e mRNA from degradation by ribonucleases.,Functions of the poly(A) tail poly(A)尾的功能,AAAAAAA - - - - - - AAAA,There is also some evidence that the poly-A tail is involved in splicing and enhances translation of mRNAs.,5.3 Splicing / 剪接,Exons: Parts of a gene that are expressed as protein. Introns: Se
11、quences that do not code for protein and interrupt the coding regions. Splicing: The process of removing introns from a pre-mRNA.,5.3.1 The Basic Splicing Reaction 基本的剪接反应,5 AG/GUAUGUbody of intronUACUAAC-YAG / 3,Splice sites in yeast,Splice sites: Sequences that mark the beginning and ends of intro
12、ns and exons.,The basic splicing reaction,5.3.2 Proteins involved in Splicing 在剪接中发挥作用的蛋白质,Spliceosome: The collection of factors, especially snRNPs, that help with the splicing of introns.,snRNPs: small nuclear ribonucleoproteins,snRNPs are small particles found in the nucleus and contain both prot
13、ein and RNA. snRNPs functioning in splicing: U1, U2, U4, U5 and U6.,Now showing mRNA splicing,File: biophoto7 Lehninger Principles of Biochemistry mRNA splicing,5.3.3 Self-Splicing / 自我剪接,Tetrahymena thermophilia 嗜热四膜虫,Splicing that occurs without the help of proteins or snRNPs is called self-splici
14、ng.,Self-splicing of group I intron,Self-splicing of group II intron II 类内含子的自我剪接,With help of proteins,Without help of proteins,Self-splicing,Normal splicing,5.3.4 Trans-Splicing / 反式剪接,Mature mRNA,Protein,RNA piece of separate origin,DNA,Trans-Splicing / 反式剪接,5.3.5 Reasons for Introns 内含子存在的原因,Int
15、rons-early theory,Introns-late theory,Intron: “Why am I here?”,Intron-early theory / 内含子早现说,E. coli,“I heard somebody say E. coli once had introns. Is that true?”,“Could be. Who knows?!”,Intron-late theory / 内含子后现说,E. coli,“We are the ones having introns! We are proud of it.”,Alternative splicing / 可变剪接,Alternative splicing: a kind of splicing that can produce various proteins fr
