《分子生物学》2cha.ppt

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1、Chapter 7Chromosomes,chromatin,and the nucleosomeNucleus:细胞核细胞核;Nucleolus:核仁核仁Nucleoid:类核类核Mitosis:有丝分裂；有丝分裂；Meiosis：减数分裂：减数分裂interphase：分裂间期：分裂间期Histone:组蛋白；组蛋白；Nucleosome:核小体核小体Chromosome:染色体；染色体；Chromatin:染色质；染色质；eu-;hetero-Centromere(中心粒）中心粒）Telomere（端粒）（端粒）Repetitive DNA (重复重复DNA)Tandem gene cl

2、uster（串联基因（串联基因 簇）簇）VocabularyCharacteristics of Eukaryotic Genome0.414.761021703060Length(cm)3.0109 10bp 3.4nm/helix=102cmThe importance of packing of DNA into chromosomesChromosome is a compact form of the DNA that readily fits inside the cellTo protect DNA from damageDNA in a chromosome can be tr

3、ansmitted efficiently to both daughter cells during cell divisionChromosome confers an overall organization to each molecule of DNA,which facilitates gene expression as well as recombinationChromosomeThe nucleosomeHigher-order chromatin structureRegulation of chromatin structureNucleosome assemblyDN

4、AProteinChromosomes:DNAShape:circular or linear Number in an organism is characteristic Copy:haploid,diploid,polyploidproblemGenome size:the length of DNA associated with one haploid complement of chromosomes Gene number:the number of genes included in a genome Gene density:the average number of gen

5、es per Mb of genomic DNAGenome&the complexity of the organism Genome&the complexity of the organism Genome size/Gene Density:1.Increases in gene size:a.increase in the sequence of regulatory sequence;b.presence of introns(splicing)a.unique;b.repeated2.Increases in the DNA between genes(intergenic se

6、quences):Pseudogenes are inactive but stable components of the genome derived by mutation of an ancestral active gene.Pseudogenethe formation of pseudogenethe formation of pseudogeneFigure 6.8 the formation of truncated gene(截短基因)and gene fragment.tandem repetitive DNADNA Satellite DNA:Satellite DNA

7、:Minisatellite DNAMinisatellite DNA Microsatellite DNA Microsatellite DNADispersed repetitive DNA Dispersed repetitive DNA 产生原因产生原因产生原因产生原因转座：转座：遗传元件在遗传元件在遗传元件在遗传元件在DNADNADNADNA中从一个位置移动到另一个位置的过程中从一个位置移动到另一个位置的过程中从一个位置移动到另一个位置的过程中从一个位置移动到另一个位置的过程 RNARNA介导介导真核存在真核存在/原核无原核无 机制：机制：RetrotranspositionRetr

8、otransposition （反转录转座）反转录转座）反转录转座）反转录转座）反转录元件反转录元件反转录元件反转录元件 DNADNA转座子转座子原核常见原核常见/真核少真核少 机制：机制：复制型和保守型复制型和保守型 水平基因转移水平基因转移 3.The Repetitive DNA content of Genomes (重复DNA序列)Figure6.15 Satellite DNA from the Figure6.15 Satellite DNA from the human genomehuman genomeSatellite DNA(卫星卫星DNA)DNA):核心序列为核心序列

9、为5200bp，主要存在，主要存在于染色体着丝粒区，于染色体着丝粒区，Centrifugation(离心分离技术)常用的两类离心分离方法是常用的两类离心分离方法是 差速离心差速离心差速离心差速离心(differential centrifugation)(differential centrifugation)密度梯度密度梯度密度梯度密度梯度离心离心离心离心(density gradient centrifugation)(density gradient centrifugation)离心分离细胞组分和生物分子是最常用的分离方法蔗糖密度梯度离心分离溶酶体、线粒体和微粒体蔗糖密度梯度离心分离溶

10、酶体、线粒体和微粒体CsCl 密度梯度离心分离密度梯度离心分离DNAFigure6.15 Satellite DNA from the Figure6.15 Satellite DNA from the human genomehuman genomeSatellite DNA(卫星卫星DNA)DNA):核心序列为核心序列为5200bp，主要存在，主要存在于染色体着丝粒区，于染色体着丝粒区，Minisatellite DNA(小卫星DNA)核心序列为核心序列为25bp 40 kb)4.Composed of largely repetitive DNA sequencesCentromeres

11、Required for the correct segregation of the chromosomes after replicationChromosomeDNAProteinlThe majority of the associated proteins are small,basic proteins called histones.lOther proteins associated with the chromosome are referred to as non-histone proteins,including numerous DNA binding protein

12、s that regulate the transcription,replication,repair and recombination of DNA.Half of the molecular mass of eukaryotic chromosome is proteinChromosomeDNAProteinThe nucleosomenDiscoverynComponentsnAtomic structureNucleosomes1.1973-1974:Nuclease protection analysis 2.1974:electron micrographs 3.Bioche

13、mistry analysis:Component:eight histone proteins （H2A，H2B，H3，H4）+140150bp DNANuclease protein analysis of chromatin from hunman nuleiNuclease protein analysis of chromatin from hunman nuleiThe nucleosome componentsn nDNAn nProteinscore DNA:147 bplinker DNA:20-60 bpHistonesFive abundant histonesStruc

14、tural foldN-terminal“tail”Histones are small,positively charged(basic)proteinsFigure 7-19 The core histones share a common structural foldhistone-fold domainthe sites of extensive modificationsH2BH2AH3H4a2a3a1a2a3a1a2a3a1a2a3a1L1L2aNH3H44 helix-bundleH3H4H3H44 helix-bundleH4H2BH2AH4H2BH2AH3H4H2BH2AH

15、3Diameter=110 AH4H2BH2AH3Width=45 AChromosomeDNAProteinThe nucleosomenDiscoverynComponentsnAtomic structureHydrogen bonds:histone (140)DNA:phosphodiester backbone 7:Bases1.sequence-independent contacts2.The histone N-terminal tailsMinor grooveThe histone tails emerge from the core of the nucleosome

16、at specific positions,serving as the grooves of to direct the DNA wrapping around the histone core in a left-handed manner.a screwChromosomeThe nucleosomeDNAProteinHigher-order chromatin structurenHistone H1 binds to the linker DNA n30-nm fiber nFurther compaction of DNAnHistone variantsHistone H1 b

17、inds to the linker DNAn nH1 binds two distinct regions of DNA 1.one end of the nucleosome 2.central DNA helix of the nucleosomenThe addition of Histone H1 lead to more compact nucleosomal DNAthe 30-nm fiber6 nuleosomes/superhelixLinker DNA circles around the central axisLinker DNA pass through the c

18、entral axisFurther compaction of DNA involves large loops of nucleosomal DNA llAdditional 10Additional 103 3-10-104 4-fold compaction is-fold compaction is required,but the mechanism is unclearrequired,but the mechanism is unclearllThe nuclear scaffold model is proposedThe nuclear scaffold model is

19、proposedThe higher-order structure of chromotinHistone variantsll Several histone variants are Several histone variants are found in eukaryotesfound in eukaryotesll This variants can replace one of This variants can replace one of the 4 standard histones to form the 4 standard histones to form alter

20、nate nucleosomesalternate nucleosomesH2A.Z CENP-A is associated with the nucleosomes containing CENP-A is associated with the nucleosomes containing centromeric DNAcentromeric DNAFigures 7-33 Alteration of chromatin by incorporation of histone variantsH3ChromosomeThe nucleosomeHigher-order chromatin

21、 structureRegulation of chromatin structure1.Nucleosome positioning2.Modification of the N-terminal tails of the histonesDNA Must Be AccessedThe interaction of DNA with the histone octamer is dynamic Nucleosome remodeling complexesA large protein complexes facilitate A large protein complexes facili

22、tate changes in nucleosome location or changes in nucleosome location or interaction with the DNA using the interaction with the DNA using the energy of ATP hydrolysis.energy of ATP hydrolysis.Fig 7-35 Nucleosome movement catalyzed by nucleosome remodeling complexesremodling Nucleosome positioningn

23、nSome Nucleosome are found in special position in vivo.DNA binding proteinsParticular DNA sequencesBent DNA Binds NucleosomesModification of the N-terminal tails of the histonesModification of the histone N-terminal tails alters the function of chromatinn nReduce the overall positive charge of histo

24、ne tailsn nRestrain to form higher order chromatin structure (repressive 30nm fiber)n nGenerates the binding sites for proteinNucleosome assemblyNucleosomes are assembled immediately after DNA replication,and the assembly requires histone chaperonesThe inheritance of histones after DNA replicationInheritance of parental H3-H4 tetramers facilitates the inheritance of chromatin statesChromosomeThe nucleosomeHigher-order chromatin structureRegulation of chromatin structureNucleosome assemblyDNAProtein