大学课程英汉对照分子生物学导论教学Chapter5课件

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。Ch

2、apter 5 mRNA Modifications in Eukaryotes第5章 真核生物mRNA的修饰5.1 Capping5.2 Polyadenylation5.3 Splicing5.4 mRNA Editing5.5 Experiments5.1 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 not yet ready for t

3、ranslation 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 bond5-5 triphosphate bondRNA

4、polymerase IIDiscussed in Chapter 4Capping takes place quite early.5-CapRNA polymerase IIRNADNACapping process / 加帽过程CTD: C-terminal domainRNA polymerase IIDNAMethyl transferaseGuanylyltransferaseRNA triphosphataseFunctions of the cap structure帽结构的功能1. Helps prevent degradation 帮助防止降解2. Helps transp

5、ort 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RNaseRNase5-5 triphosphate bond5-3 phosphodiester bond5-3 phosphodiester bond2. Helps transport into cytoplasm帮助转运到细胞质中Discussed in Chapter 73. Enhances translation

6、/ 增强转译Cap-binding proteinNo translation occurs.RNA polymerase IIRNADNA4. Helps remove the first intron帮助去除第一个内含子The first intronAAAAAAA - - - - - - AAAA5.2 Polyadenylation / 聚腺苷酸化Poly(A) tailPolyadenylationPre-mRNAPolyadenylation is a modification process in which a string of about 250 adenine nucle

7、otides 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 - - - - - - AAAAPoly(A) tailPolyadenylationPre-mRNAPolyadenylation signal has a typical sequence of AAUAAA. This sequence gives an instruction as “to cut the m

8、RNA about 20 nucleotides downstream, near a GU-rich sequence”. AAUAAAGUThe cleavage complex / 切割复合体CleavagecomplexCPSF (cleavage andpolyadenylationspecificity factor)CstF (cleavagestimulation factor)CFI (cleavage factor I)CFII (cleavage factors II)Poly(A)-Binding ProteinCTD: C-terminal domainProtein

9、sfor cappingat CTDProteins forpolyadenylationat CTDFunctions of the poly(A) tailpoly(A)尾的功能AAAAAAA - - - - - - AAAAAAAAAAA - - -AAAAAAThe main function of poly(A) tail is to protect the mRNA from degradation by ribonucleases.Functions of the poly(A) tailpoly(A)尾的功能AAAAAAA - - - - - - AAAAThere is al

10、so 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: Sequences that do not code for protein and interrupt the coding regions. Splicing: The process of removing introns from a p

11、re-mRNA.5.3.1 The Basic Splicing Reaction基本的剪接反应5 AG/GUAUGUbody of intronUACUAAC-YAG / 3Splice sites in yeastSplice sites: Sequences that mark the beginning and ends of introns and exons.The basic splicing reaction5.3.2 Proteins involved in Splicing在剪接中发挥作用的蛋白质Spliceosome: The collection of factors,

12、 especially snRNPs, that help with the splicing of introns. snRNPs: small nuclear ribonucleoproteinssnRNPs are small particles found in the nucleus and contain both protein and RNA. snRNPs functioning in splicing: U1, U2, U4, U5 and U6.5.3.3 Self-Splicing / 自我剪接Tetrahymena thermophilia嗜热四膜虫 Splicing

13、 that occurs without the help of proteins or snRNPs is called self-splicing. tetrhaimn :mfaili Self-splicing of group I intronSelf-splicing of group II intronII 类内含子的自我剪接With helpof proteinsWithout helpof proteinsSelf-splicingNormalsplicing5.3.4 Trans-Splicing / 反式剪接Mature mRNAProteinRNA piece ofsep

14、arate origin DNATrans-Splicing / 反式剪接5.3.5 Reasons for Introns内含子存在的原因Introns-early theoryIntrons-late theoryIntron: “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?”“Couldbe. Who knows?!”Intron-late theory / 内含子后现说E. coli“We are the on

15、eshaving introns! We are proud of it.”Alternative splicing / 可变剪接Alternative splicing: a kind of splicing that can produce various proteins from one gene.Drosophila Dscam geneExon 3Exon 54.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12Exon 4 Exon 6 Exon 9Alternative Splicing Goes Mainstream可变剪接成为主流The Scientist, Volume 17(24): 28, Dec. 15, 2003 drusfil Alternative splicing / 可变剪接5.4 mRNA Editing / mRNA编辑Sleeping sickness is one of the most neglected human diseases, and mainly affects the most deprived peoples of some African countries. Trypan