分子生物学课件第五章

1 / 48In 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 。Chapter 5 mRNA Modifications in Eukaryotes第 5章 真核生物 mRNA的修饰2 / 485.1 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3 / 485.1 Capping / 加帽An mRNA that has been transcribed but is not 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。4 / 48Structure of the cap / 帽的结构5-3 phospho-diester bond5-5 triphosphate bond5 / 48RNA polymerase IIDiscussed in Chapter 46 / 48Capping takes place quite early.5-CapRNA polymerase IIRNADNA7 / 48Capping process / 加帽过程8 / 48CTD: C-terminal domainRNA polymerase IIDNAMethyl transferaseGuanylyltransferaseRNA triphosphatase9 / 48Functions of the cap structure帽结构的功能1. Helps 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10 / 481. Helps prevent degradation帮助防止降解RNase RNase5-5 triphosphate bond5-3 phosphodiester bond5-3 phosphodiester bond11 / 482. Helps transport into cytoplasm帮助转运到细胞质中Discussed in Chapter 712 / 483. Enhances translation / 增强转译Cap-binding proteinNo translation occurs.13 / 48RNA polymerase IIRNADNA4. Helps remove the first intron帮助去除第一个内含子The first intron14 / 48AAAAAAA - - - - - - AAAA5.2 Polyadenylation / 聚腺苷酸化Poly(A) tailPolyadenylationPre-mRNAPolyadenylation is a modification process in which a string of about 250 adenine nucleotides is added to the 3 end of the transcript.15 / 48Polyadenylation does not occur at the natural end 16 / 48The polyadenylation signal / 聚腺苷酸化信号AAAAAAA - - - - - - AAAAPoly(A) tailPolyadenylationPre-mRNAPolyadenylation signal has a typical typical sequence of AAUAAA. This sequence gives an instruction as “to cut the mRNA about 20 nucleotides downstream, near a GU-rich sequence”. AAUAAA GU17 / 48The 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 Protein19 / 48CTD: C-terminal domainProteinsfor cappingat CTDProteins forpolyadenylationat CTD20 / 48Functions 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.21 / 48Functions of the poly(A) tailpoly(A)尾的功能AAAAAAA - - - - - - AAAAThere is also some evidence that the poly-A tail is involved in splicing and enhances translation of mRNAs. 22 / 485.3 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 intronsfrom a pre-mRNA.23 / 485.3.1 The Basic Splicing Reaction基本的剪接反应5 AG/GUAUGUbody of intronUACUA AC-YAG / 3Splice sites in yeastSplice sites: Sequences that mark the beginning and ends of introns and exons.24 / 48The basic splicing reaction25 / 485.3.2 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. 26 / 48snRNPs: 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 U6.29 / 48Now showingmRNA splicingFile: biophoto7