《分子生物学》2cha.ppt
Chapter 7,Chromosomes,chromatin,and the nucleosome,Nucleus:细胞核;Nucleolus:核仁Nucleoid:类核Mitosis:有丝分裂；Meiosis：减数分裂interphase：分裂间期Histone:组蛋白；Nucleosome:核小体Chromosome:染色体；Chromatin:染色质；eu-;hetero-Centromere(中心粒）Telomere（端粒）Repetitive DNA(重复DNA)Tandem gene cluster（串联基因 簇）,Vocabulary,Characteristics of Eukaryotic Genome,The importance of packing of DNA into chromosomes,Chromosome is a compact form of the DNA that readily fits inside the cellTo protect DNA from damageDNA in a chromosome can be transmitted efficiently to both daughter cells during cell divisionChromosome confers an overall organization to each molecule of DNA,which facilitates gene expression as well as recombination,ChromosomeThe nucleosomeHigher-order chromatin structureRegulation of chromatin structureNucleosome assembly,Chromosomes:DNA,Shape:circular or linear Number in an organism is characteristic Copy:haploid,diploid,polyploid,problem,Genome 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 genes per Mb of genomic DNA,Genome&the complexity of the organism,Genome&the complexity of the organism,Genome size/Gene Density:,1.Increases in gene size:,increase in the sequence of regulatory sequence;presence of introns(splicing),unique;repeated,2.Increases in the DNA between genes(intergenic sequences):,Pseudogenes are inactive but stable components of the genome derived by mutation of an ancestral active gene.,Pseudogene,the formation of pseudogene,Figure 6.8 the formation of truncated gene(截短基因)and gene fragment.,tandem repetitive DNA Satellite DNA:Minisatellite DNA Microsatellite DNADispersed repetitive DNA 产生原因转座：遗传元件在DNA中从一个位置移动到另一个位置的过程 RNA介导真核存在/原核无 机制：Retrotransposition（反转录转座）反转录元件 DNA转座子原核常见/真核少 机制：复制型和保守型 水平基因转移,3.The Repetitive DNA content of Genomes(重复DNA序列),Figure6.15 Satellite DNA from the human genome,Satellite DNA(卫星DNA):核心序列为5200bp，主要存在于染色体着丝粒区，,Centrifugation(离心分离技术),常用的两类离心分离方法是 差速离心(differential centrifugation)密度梯度离心(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,CsCl 密度梯度离心分离DNA,Figure6.15 Satellite DNA from the human genome,Satellite DNA(卫星DNA):核心序列为5200bp，主要存在于染色体着丝粒区，,Minisatellite DNA(小卫星DNA)核心序列为25bp，主要存在于染色体靠近端粒处,与DNA复制有关染色体其他位置的小卫星DNA,功能不清,DNA复制时微卫星拷贝有时发生滑移,导致插入或缺失一个或多个重复单位,使核心序列重复数目发生变化，以至于任何两个不同个体间中都存在串联数目的差异-可用于DNA Finger(DNA 指纹)可用来研究人群相关性,Microsatellite DNA(微卫星DNA),指以27个碱基为核心单位串联重复而成的一类序列，又称为STR short tandem repeat(短串联重复序列）,Microsatellites are useful for genetic mapping,Origins of replicationCentromeresTelomeres,Critical DNA elements in Chromosome duplication&segregation,Figure 7-6 Centromeres,origin of replication and telomere are required for eukaryotic chrom.maintenance,Mitotic chromosome at metaphase,Kinetochore动粒,Centromere着丝粒,Centromere:着丝粒,a region of chromosome(染色体)where two sister chromatids(染色单体)are joined at metaphase.2.Kinetochore:动粒,a protein complex attaching microtubules(微管)of mitotic spindle(纺锤体)3.Telomere:端粒,end of chromosome,Mitotic spindle,Telomere 端粒,Direct the formation of kinetochore(an elaborate protein complex)essential for chromosome segregationOne chromosome,one centromereThe size varies(200 bp-40 kb)Composed of largely repetitive DNA sequences,Centromeres,Required for the correct segregation of the chromosomes after replication,Chromosome,The majority of the associated proteins are small,basic proteins called histones.Other proteins associated with the chromosome are referred to as non-histone proteins,including numerous DNA binding proteins that regulate the transcription,replication,repair and recombination of DNA.,Half of the molecular mass of eukaryotic chromosome is protein,Chromosome,The nucleosome,DiscoveryComponentsAtomic structure,Nucleosomes,1973-1974:Nuclease protection analysis,2.1974:electron micrographs,3.Biochemistry analysis:Component:eight histone proteins（H2A，H2B，H3，H4）+140150bp DNA,Nuclease protein analysis of chromatin from hunman nulei,The nucleosome components,DNAProteins,core DNA:147 bplinker DNA:20-60 bp,Histones,Five abundant histonesStructural foldN-terminal“tail”,Histones are small,positively charged(basic)proteins,Figure 7-19 The core histones share a common structural fold,histone-fold domain,the sites of extensive modifications,H2B,H2A,H3,H4,a2,a3,a1,a2,a3,a1,a2,a3,a1,a2,a3,a1,L1,L2,aN,H3,H4,4 helix-bundle,H3,H4,H3,H4,4 helix-bundle,H4,H2B,H2A,H4,H2B,H2A,H3,H4,H2B,H2A,H3,Diameter=110 A,H4,H2B,H2A,H3,Width=45 A,Chromosome,The nucleosome,DiscoveryComponentsAtomic structure,Hydrogen bonds:histone(140)DNA:phosphodiester backbone 7:Bases,1.sequence-independent contacts,2.The histone N-terminal tails,Minor groove,The histone tails emerge from the core of the nucleosome at specific positions,serving as the grooves of to direct the DNA wrapping around the histone core in a left-handed manner.,a screw,ChromosomeThe nucleosome,Higher-order chromatin structure,Histone H1 binds to the linker DNA 30-nm fiber Further compaction of DNAHistone variants,Histone H1 binds to the linker DNA,H1 binds two distinct regions of DNA,one end of the nucleosome central DNA helix of the nucleosome,The addition of Histone H1 lead to more compact nucleosomal DNA,the 30-nm fiber,6 nuleosomes/superhelixLinker DNA circles around the central axis,Linker DNA pass through the central axis,Further compaction of DNA involves large loops of nucleosomal DNA,Additional 103-104-fold compaction is required,but the mechanism is unclearThe nuclear scaffold model is proposed,The higher-order structure of chromotin,Histone variants,Several histone variants are found in eukaryotes This variants can replace one of the 4 standard histones to form alternate nucleosomes,H2A.Z,CENP-A is associated with the nucleosomes containing centromeric DNA,Figures 7-33 Alteration of chromatin by incorporation of histone variants,H3,ChromosomeThe nucleosomeHigher-order chromatin structure,Regulation of chromatin structure,Nucleosome positioningModification of the N-terminal tails of the histones,DNA Must Be Accessed,The interaction of DNA with the histone octamer is dynamic,Nucleosome remodeling complexes,A large protein complexes facilitate changes in nucleosome location or interaction with the DNA using the energy of ATP hydrolysis.,Fig 7-35 Nucleosome movement catalyzed by nucleosome remodeling complexes,remodling,Nucleosome positioning,Some Nucleosome are found in special position in vivo.,DNA binding proteinsParticular DNA sequences,Bent DNA Binds Nucleosomes,Modification of the N-terminal tails of the histones,Modification of the histone N-terminal tails alters the function of chromatin,Reduce the overall positive charge of histone tailsRestrain to form higher order chromatin structure(repressive 30nm fiber)Generates the binding sites for protein,Nucleosome assembly,Nucleosomes are assembled immediately after DNA replication,and the assembly requires histone chaperones,The inheritance of histones after DNA replication,Inheritance of parental H3-H4 tetramers facilitates the inheritance of chromatin states,ChromosomeThe nucleosomeHigher-order chromatin structureRegulation of chromatin structureNucleosome assembly,
