分子生物学教学课件：CHAPTER 7 Chromosomes, chromatin,

1、回顾上一节的内容回顾上一节的内容1. 什么是分子生物学？什么是分子生物学？2. 分子生物学发展的三个阶段：分子生物学发展的三个阶段： 1）准备和酝酿阶段：）准备和酝酿阶段：产生了两点对生命本质的认识上的重大突破。 证明DNA是遗传物质的两个关键性实验：肺炎球菌转化实验和T4 噬菌体在大肠杆菌的实验。 2）现代分子生物学的诞生和发展：）现代分子生物学的诞生和发展：DNA双螺旋结构的提出。双螺旋结构的提出。 逐渐完善了对DNA复制机理的认识；逐步阐明了RNA转录合成的 机理；认识了蛋白质翻译合成的基本过程；补充和完善了遗传信 息传递的中心法则；对蛋白质结构与功能的进一步认识。 3）70年代后期以基

2、因工程技术的出现作为新的里程碑，标志着人类年代后期以基因工程技术的出现作为新的里程碑，标志着人类 深入认识生命本质并能改造生命的新的时期的开始。深入认识生命本质并能改造生命的新的时期的开始。 DNA重组技术；基因组研究的发展；单克隆抗体和遗传工程单克隆抗体 的产生；基因表达调控机理的逐步完善和细胞信号传导机理的深入研究。Chapter 7Chromosomes, chromatin, and the nucleosomeMolecular Biology of the Gene- James Watson et al. Part I: Chemistry and Genetics Part I

3、I: Maintenance of the Genome Part III: Expression of the GenomePart IV: RegulationPart V: MethodsCh 6: The structures of DNA and RNA Ch 7: Chromosomes, chromatins and the nucleosomeCh 8: The replication of DNACh 9: The mutability and repair of DNACh 10: Homologous recombination at the molecular leve

4、lCh 11: Site-specific recombination and transposition of DNAPart II: Maintenance of the GenomeNucleus: 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 (着丝粒）

5、；着丝粒）； Telomere（端粒）（端粒）Repetitive DNA (重复重复DNA)Tandem gene cluster（串联基因簇）（串联基因簇）VocabularyWhat Exactly is a chromosome?Chromosomes are the rod-shaped, filamentous bodies present in the nucleus, which become visible during cell division. They are the carriers of the gene or unit of heredity.Chromosom

6、eChromosome 染色体是细胞内具有遗传性质的物体，易被碱性染料染成深色而得其名，是由染色体是细胞内具有遗传性质的物体，易被碱性染料染成深色而得其名，是由DNA、蛋白质和少量、蛋白质和少量RNA组成的线状或棒状物，在细胞分裂时，染色质丝经过组成的线状或棒状物，在细胞分裂时，染色质丝经过螺旋化、折叠、包装成为染色体，为显微镜下可见的具有不同形状的小体。螺旋化、折叠、包装成为染色体，为显微镜下可见的具有不同形状的小体。What Exactly is a chromosome? Chromosomes contain both DNA and protein. Most of the prot

7、ein is small protein, histones (H2A/H2B/H3/H4). There are also non-histone proteins, including numerous DNA binding proteins that regulate the transcription, replication, repair and recombination of DNA.ChromosomeChromosome染色体是由线性双链染色体是由线性双链DNA分子同分子同蛋白质蛋白质形成的复合物。形成的复合物。Proteins in chromosomeIn eukar

8、yotic cells a given region of DNA with its associated proteins is called chromatin.Nucleosomes: regular association of DNA with histones to form a structure effectively compacting DNA.The importance of packing of DNA into chromosomesChromosome is a compact form of the DNA that readily fits inside th

9、e cell.To protect DNA from damage.DNA in a chromosome can be transmitted efficiently to both daughter cells during cell division.Chromosome confers an overall organization to each molecule of DNA, which facilitates gene expression as well as recombination.ChromosomeChromosomeChromosomeChromosome The

10、 structure of packing of DNA into chromosomesOutline1. Chromosome sequence and diversity（基因组序列和染色体多样性）（基因组序列和染色体多样性）2. The chromosome duplication and segregation（染色体的复制和分离）（染色体的复制和分离）3. The Nucleosome（核小体）（核小体）4. Higher-order chromatin structure5. Regulation of chromatin structure6. Nucleosome assem

11、bly（核小体的组装）（核小体的组装）Chromosome sequence and diversityChromosomes can be circular or linearShape: circular or linear (see P140,Table 7-1)Chromosome sequence and diversityChromosome sequence and diversityProkaryotic cell typically have only one complete copy of their chromosome. Majority of eukaryotic

12、cells are diploid; that is, they contain two copies of each chromosome. Every Cell Maintains a Characteristic Number of ChromosomesChromosome sequence and diversityChromosome sequence and diversity单倍单倍体体双倍体细胞双倍体细胞单倍体细胞单倍体细胞（精子和卵子）（精子和卵子）Figure 7-1 Comparison of typical prokaryotic and eukaryoti cell

13、Chromosome sequence and diversityChromosome sequence and diversityHomolog同源染色体同源染色体 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 DNAGenome size i

14、s related to the complexity of the organism 基因组（基因组（Genome）：指单倍体细胞中包括编码序列和非编）：指单倍体细胞中包括编码序列和非编码序列在内的全部码序列在内的全部DNA分子分子。更确切的说，。更确切的说，基因组基因组是单倍体是单倍体细胞核细胞核内的全部内的全部 DNA分子。分子。 Chromosome sequence and diversityChromosome sequence and diversityMore Genes are Required to Direct the Formation of More Complex

15、Organisms0.414.761021703060Length (cm)3.0109 10bp 3.4nm/helix= 102cmChromosome sequence and diversityChromosome sequence and diversity生物的复杂度越高所需的基因也越多。一般原核细胞的基因组不足生物的复杂度越高所需的基因也越多。一般原核细胞的基因组不足10Mb，而单，而单细胞真核生物的基因组都小于细胞真核生物的基因组都小于50Mb，多核细胞的基因组甚至超过，多核细胞的基因组甚至超过10万万Mb。 生物的复杂性与基因组的大小并不完全成比例增生物的复杂性与基因组的大小

16、并不完全成比例增加。如果蝇和蝗虫，虽然是具有相似复杂度的生物加。如果蝇和蝗虫，虽然是具有相似复杂度的生物体，但是基因组大小明显不同。体，但是基因组大小明显不同。 基因组测序的结果发现，越是复杂的生物体，参基因组测序的结果发现，越是复杂的生物体，参与编码的基因序列在整个基因组序列中所占的比例与编码的基因序列在整个基因组序列中所占的比例越少，即基因密度越低。越少，即基因密度越低。 基因组和基因的数量，随着生物体的复杂而增多，基因组和基因的数量，随着生物体的复杂而增多，但是基因的密度却降低。但是基因的密度却降低。More Complex Organisms Have Decreased Gene D

17、ensityChromosome sequence and diversityChromosome sequence and diversityMore Complex Organisms Have Decrease Gene DensityFigure 7-2. Comparison of the chromosomal gene density for different organism.基因组密度的简单衡量方法是每基因组密度的简单衡量方法是每Mb基因组基因组DNA上基因的平均数目。上基因的平均数目。 Two factors contribute to the decreased gen

18、e density in eucaryotic cells：1. Increases in gene size: 非连续性的蛋白质编码区被非连续性的蛋白质编码区被内含子内含子 (introns) 所分开。内含子的存在极大地增加了编码所分开。内含子的存在极大地增加了编码一个基因所需的一个基因所需的DNA的长度。的长度。.2. Increases in the DNA between genes（intergenic sequences，基因间序列基因间序列）: 单一的和重单一的和重复的，这些序列参与指导和调控复的，这些序列参与指导和调控DNA的的转录转录（regulatory sequences

19、，调控序列）调控序列）。随着生物体复杂度的提高，编。随着生物体复杂度的提高，编码更多的基因，调控基因表达的调控序列也更加复杂，其序码更多的基因，调控基因表达的调控序列也更加复杂，其序列大小也不断增加。列大小也不断增加。Genes Make Up Only a Small Proportion of the Eukaryotic Genome Chromosome sequence and diversityChromosome sequence and diversityChromosome sequence and diversityChromosome sequence and diver

20、sityThe organization and content of the human genomeFigure 7-4. Chromosome sequence and diversityChromosome sequence and diversityThe majority of human intergenic sequences are composed of repetitive DNAAlmost half of the human genome is composed of DNA sequences that are repeated many times in the

21、genome: Chromosome sequence and diversityChromosome sequence and diversityRepetitive DNATandem repetitive DNADispersed repetitive DNA 卫星卫星DNA(satelliteDNA)：是一类高度重复：是一类高度重复序列序列DNA在介质氯化铯中作在介质氯化铯中作密度密度梯度离心后，梯度离心后，在离心管内按其大小分布形成了不同的条带；在离心管内按其大小分布形成了不同的条带；根据荧光强度的分析，可以看到在一条主带根据荧光强度的分析，可以看到在一条主带以外还有一个或多个小的卫

22、星带。以外还有一个或多个小的卫星带。Chromosome sequence and diversityChromosome sequence and diversityCsCl 密度梯度离心分离密度梯度离心分离DNA串联重复的串联重复的DNA分散重复的分散重复的DNASatellite DNAChromosome sequence and diversityChromosome sequence and diversity卫星卫星DNA(satelliteDNA)：GC含量一般少于主带中的含量一般少于主带中的DNA，浮力密度也低。，浮力密度也低。按按其重复单元的核苷酸的多少可分为两类：一类是其

23、重复单元的核苷酸的多少可分为两类：一类是小卫星小卫星DNA，由几百个核苷酸，由几百个核苷酸对的单元重复组成；另一类是对的单元重复组成；另一类是微卫星微卫星DNA，由，由2个到个到20个左右的核苷酸对的单元个左右的核苷酸对的单元重复成百上千次所组成。关于卫星的产生机理，认为可能是重复成百上千次所组成。关于卫星的产生机理，认为可能是DNA复制和修复过复制和修复过程中的滑动错配或染色体减数分裂时姊妹染色单体不均程中的滑动错配或染色体减数分裂时姊妹染色单体不均等交换的结果。等交换的结果。Satellite DNA consists of very large arrays of tandemly re

24、peating, non-coding DNA. Satellite DNA is the main component of functional centromeres, and form the main structural constituent of heterochromatin. 核心序列为核心序列为25bp 25bp ，主要存在于染色体靠近端粒处，主要存在于染色体靠近端粒处, ,与与DNADNA复制有关。染色体其他位置的小卫星复制有关。染色体其他位置的小卫星DNA,DNA,功能不清。功能不清。Chromosome sequence and diversityChromosom

25、e sequence and diversityMicrosatellite DNA (微卫星微卫星DNA)指以指以2 27 7个碱基为核心单位串联重复而成的个碱基为核心单位串联重复而成的一类序列，又称为一类序列，又称为STR short tandem STR short tandem repeat(repeat(短串联重复序列）短串联重复序列）Chromosome sequence and diversityChromosome sequence and diversityA class of miniaturized satellite 微卫星在基因组中的功能尚不清楚，可能参与遗传物微卫星

26、在基因组中的功能尚不清楚，可能参与遗传物质的结构改变，基因调控及细胞分化等过程，有自身质的结构改变，基因调控及细胞分化等过程，有自身特异结合蛋白，尚能直接编码蛋白质。如脆性特异结合蛋白，尚能直接编码蛋白质。如脆性X综合综合症基因内症基因内(CGG)n串联重复，编码串联重复，编码30个精氨酸；个精氨酸；(CAG)n是人类基因库是人类基因库DNA序列外显子中发现最多的序列外显子中发现最多的一种。另外，微卫星在促进染色体凝集，维持染色体一种。另外，微卫星在促进染色体凝集，维持染色体结构等方面也有作用，可参与染色体折叠、染色体端结构等方面也有作用，可参与染色体折叠、染色体端粒形成等。粒形成等。(CA/

27、GT)n、(GATA)n可能与致育性、性可能与致育性、性别分化、别分化、X染色体失活有关。染色体失活有关。 Chromosome sequence and diversityChromosome sequence and diversityOutline1. Chromosome sequence and diversity2. The chromosome duplication and segregation3. The Nucleosome4. Higher-order chromatin structure5. Regulation of chromatin structure6. N

28、ucleosome assemblyThe chromosome duplication and segregationChromosome duplication & segregationChromosome duplication & segregationEukaryotic chromosomes require centromeres, telomeres and origins of replication to be maintained during cell division.These elements are not involved in gene e

29、xpression，but all of these features are critical for the proper duplication and segregation of the chromosomes during cell division.Figure 7-6 Centromeres, origin of replication, and telomere are required for eukaryotic chromosome maintenance.Chromosome duplication & segregationChromosome duplic

30、ation & segregation Sites at which the DNA replication machinery assembles to initiate replication; required for replication 30-40 kb apart on each eukaryotic chromosome Only one origin for prokaryotic chromosomeOrigins of ReplicationChromosome duplication & segregationChromosome duplication

31、 & segregation复制起始位点是复制起始位点是DNA复制机器组装和复制起始的位点。复制机器组装和复制起始的位点。Required for the correct segregation of the chromosomes after DNA replication. Direct the formation of kinetochore (an elaborate protein complex) essential for chromosome segregation. One chromosome, one centromere The size varies (酵母中

32、少于酵母中少于200 bp；真核细真核细胞中大于胞中大于40 kb) Composed of largely repetitive DNA sequences (Satellite DNA) .CentromeresChromosome duplication & segregationChromosome duplication & segregationFigure 7-7 More or less than one centromere leads to chromasome loss or breakage.中着丝粒中着丝粒亚中着丝粒亚中着丝粒亚端部着丝粒亚端部着丝粒端

33、部着丝粒端部着丝粒CentromeresCentromere (着丝粒着丝粒): a region of chromosome where two sister chromatids are joined at metaphase.Mitotic chromosome at metaphaseKinetochore动粒动粒Centromere着丝粒着丝粒 Kinetochore (动粒动粒): a protein complex attaching microtubules(微管微管) of mitotic spindle (纺锤体纺锤体).Mitotic spindle（有丝分裂（有丝分裂纺

34、锤体）纺锤体）Telomere 端粒端粒 Chromosome duplication & segregationChromosome duplication & segregationKinetochore (动粒动粒):由多种由多种蛋白质在有丝分裂染色体着丝蛋白质在有丝分裂染色体着丝粒部位形成的一种圆盘状结构。粒部位形成的一种圆盘状结构。微管与之连接，与染色体分离微管与之连接，与染色体分离密切相关。每一个中期染色体密切相关。每一个中期染色体含有两个动粒，位于着丝粒的含有两个动粒，位于着丝粒的两侧。两侧。 间期间期前期前期中期中期后期后期末期末期胞质分裂胞质分裂Centrom

35、eres direct the formation of an elaborate protein complex, in this case, called a kinetochore.动粒的作用机制是着丝粒动粒的作用机制是着丝粒DNA与蛋白质纤维（微管）相互作与蛋白质纤维（微管）相互作用，拉动姐妹染色体相互远离用，拉动姐妹染色体相互远离并进入两个子细胞并进入两个子细胞. Kinetochore (动粒动粒):A telomere is a region of repetitive DNA sequence at the end of a chromosome, which protects

36、 the end of the chromosome from deterioration or from fusion with neighbouring chromosomes. Chromosome duplication & segregationChromosome duplication & segregationTelomere (端粒端粒)端粒是染色体末端的DNA重复序列，作用是保持染色体的完整性。细胞分裂一次，由于DNA复制时的方向必须从5方向到3方向，DNA每次复制端粒就缩短一点（参见冈崎片段）。一旦端粒消耗殆尽，染色体则易于突变而导致动脉硬化和某些癌症。因

37、此，端粒和细胞老化有明显的关系。一个基因组内的所有端粒都是由相同的重复序列组成，但不同物种的端粒的重复序列是不同的。Function of TelomeresChromosome duplication & segregationChromosome duplication & segregation保护染色体不被核酸酶降解；保护染色体不被核酸酶降解；防止染色体相互融合；防止染色体相互融合；为端粒酶提供底物，解决为端粒酶提供底物，解决DNA复制的末复制的末端隐缩，保证染色体的完全复制。端隐缩，保证染色体的完全复制。端粒、着丝粒和复制原点是染色体保持端粒、着丝粒和复制原点是染色体

38、保持完整和稳定的三大要素。完整和稳定的三大要素。Cell cycle: a single round of cell division.Mitotic cell division: Most eukaryotic cell divisions maintain the number is chromosomes in the daughter cells that were present in the parental cell. This type of division is called mitotic cell division.Eukaryotic chromosome dupli

39、cation & segregation occur in separate phases of the cell cycle Chromosome duplication & segregationChromosome duplication & segregationcellCycle.swfFigure 7-10 The eukaryotic mitotic cell cycle细胞完成一轮分裂所需要的过程叫细胞周期。大多数真核细胞的分裂会使子细胞细胞完成一轮分裂所需要的过程叫细胞周期。大多数真核细胞的分裂会使子细胞维持与母细胞一致的染色体数目叫细胞的有丝分裂。维

40、持与母细胞一致的染色体数目叫细胞的有丝分裂。Figure 7-11 The events of S phasechromatidCohesion(黏粒黏粒)：复制后的姐妹染色单体通过黏粒的分子聚复制后的姐妹染色单体通过黏粒的分子聚在一起，即姐妹染色单体的附着。这种状态一直维持到染色单体的在一起，即姐妹染色单体的附着。这种状态一直维持到染色单体的相互分离。相互分离。03a_function_cohesin.swfFigure 7-12 The events of M phaseMitotic spindle染色单体的配对由组装在每个着丝粒处的动粒所介导。黏粒被水解后，抵抗有丝染色单体的配对由组装

41、在每个着丝粒处的动粒所介导。黏粒被水解后，抵抗有丝分裂纺锤体的拉力消失，姐妹染色单体被迅速地拉向有丝分裂纺锤体相反的两极。分裂纺锤体的拉力消失，姐妹染色单体被迅速地拉向有丝分裂纺锤体相反的两极。M phase: condensed state, completely disentangled from each other.G1, S, G2 phases: diffused, significantly less compact. The structure of chromosome changes, e.g. DNA replication requires the nearly com

42、plete disassembly and reassembly of the proteins associated with each chromosome.Chromosome Structure Changes as Eukaryotic Cells Divide Chromosome duplication & segregationChromosome duplication & segregationChromosome condensationFigure 7-13 Changes in chromatin structureREMEMBER: chromoso

43、me is a consistently changing structure (dynamics)Figure 7-14 Model for the structure and function of cohesins and pare_meiosis_mitosis.swf有丝分裂维持亲本有丝分裂维持亲本染色体的数目；真染色体的数目；真核细胞的另一种类核细胞的另一种类型是减数分裂。与型是减数分裂。与有丝分裂不同的是有丝分裂不同的是进入减数分裂的细进入减数分裂的细胞必需是二倍体，胞必需是二倍体，染色体进行两轮分染色体进行两轮分离，最终和母细胞离，最终和母细胞相比，产生了半数相比，产生了半数的

44、染色体。的染色体。间期间期前期前期中期中期后期后期末期末期胞质分裂胞质分裂减数分裂间期减数分裂间期减数分裂减数分裂I期期减数分裂减数分裂II期期The gap phase of the cell cycle allow time to prepare for the next cell cycle stage while also checking that the previous stage is finished correctly. 细胞周期的间期有两个作用：一是为下一个细细胞周期的间期有两个作用：一是为下一个细胞周期阶段作好时间上的准备，二是检查上一胞周期阶段作好时间上的准备，二是检

45、查上一个细胞周期阶段是否正确完成。如果在细胞周个细胞周期阶段是否正确完成。如果在细胞周期的前一阶段存在问题，细胞周期检查点期的前一阶段存在问题，细胞周期检查点(cell cycle checkpoint)则停止细胞周期并给细胞提则停止细胞周期并给细胞提供时间以完成这一步。供时间以完成这一步。 Chromosome duplication & segregationChromosome duplication & segregationcontrol_cell_cycle.swfDifferent levels of chromosome structure can be obs

46、erved by microscopyFluorescent microscope image of chromosomesChromosome duplication & segregationChromosome duplication & segregationFigure 7-17 Forms of chromotin structure seen in EM (electron microscopy)Outline1. Chromosome sequence and diversity2. The chromosome duplication and segregat

47、ion3. The Nucleosome4. Higher-order chromatin structure5. Regulation of chromatin structure6. Nucleosome assemblyThe NucleosomeChromosomeDNAProteinl The majority of the associated proteins are small, basic proteins called histones. l Other proteins associated with the chromosome are referred to as n

48、on-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 proteinHigher-order Chromatin StructureHigher-order Chromatin StructureNucleosomes are the building blocks

49、 of chromosomesHigher-order Chromatin StructureHigher-order Chromatin StructureFigure 7-18 DNA packaged into Nucleosomes. (a) Schemic of the packaging and organization of nucleosomes. (b) Crystal structure of a nucleosome showing DNA wrapped around the histone Protein core. H2B is shown in red, H2A

50、in yellow, H3 in purple, and H4 in green. The NucleosomeThe The NucleosomeNucleosomeThe nucleosomeLinker DNAThe nucleosome is composed of a core of eight histone proteins and the DNA (core DNA, 146 bp) wrapped around them. The DNA between each nucleosome is called a linker DNA. Each eukaryote has a

51、characteristic average linker DNA length (20-60 bp).Figure 7-18 DNA packaged into nucleosomeNucleosome core particle The The NucleosomeNucleosomeThe The nucleosomenucleosomeBox 7-1 Figure 7-1 Progressive digestion of nucleosomal DNA with Mnase. As indicated, the nucleosome core particle is released

52、from chromatin by digestion of the linker DNA with a nuclease.Micrococcal Nuclease是一种内切核酸酶，是一种内切核酸酶，作用于单链及双链核酸，生成作用于单链及双链核酸，生成3磷酸末端。磷酸末端。 The The nucleosomenucleosomeLinker DNA is double-stranded DNA in between two nucleosome cores that, in association with histone H1, holds the cores together. Link

53、er DNA is not associated with the transcribable DNA wrapped around the core. 任何细胞中都含有没有包装成的长任何细胞中都含有没有包装成的长DNA，虽然这些，虽然这些DNA没有直接没有直接与核小体结合，但是可以结合非组蛋白，从而参与调控基因的表达、与核小体结合，但是可以结合非组蛋白，从而参与调控基因的表达、复制或重组功能等。复制或重组功能等。Histones Are Small, Positive-Charged proteinsThe The nucleosomenucleosome Five abundant his

54、tones are H1 (linker histone, 20 KDa), H2A, H2B, H3 and H4 (core histones, 11-15 KDa). The core histones share a common structural fold, called histone-fold domain. The core histones each have an N-terminal “tail”, the sites of extensive modifications.Histones are small, positively charged (basic) p

55、roteinsThe The nucleosomenucleosomeThe core histones share a common structural foldThe The nucleosomenucleosomeThe The nucleosomenucleosomeThe The nucleosomenucleosome The high affinity of the nucleosome for DNA, The distortion of the DNA when bound to the nucleosome, The lack of DNA sequence specif

56、icity The The nucleosomenucleosomeThe features of interaction between histone and DNAThe The nucleosomenucleosomeThe The nucleosomenucleosomeThe The nucleosomenucleosomeThe finding that all the sites of contact between the histones and the DNA involve either the minor groove or the phosphate backbon

57、e is consistent with the non-sequence-specific nature of the association of the histone octamer with DNA.DNA sequence-independent contacts (?) mediate interaction between the core histones and DNA Many DNA Sequence-Independent Contacts Mediate the Interaction between the Core Histones and DNAThe The

58、 nucleosomenucleosome每次每次DNADNA的小沟面对组蛋白八聚体时可发现一个接触的小沟面对组蛋白八聚体时可发现一个接触位点，共发现位点，共发现1414个不同的位点。个不同的位点。组蛋白与组蛋白与DNADNA之间是通过大量的氢键（约之间是通过大量的氢键（约4040个）所介个）所介导的。导的。这些氢键大多位于蛋白质和这些氢键大多位于蛋白质和DNADNA小沟附近磷酸二酯骨小沟附近磷酸二酯骨架上的氧原子之间。架上的氧原子之间。只有只有7 7个氢键位于蛋白质侧链和个氢键位于蛋白质侧链和DNADNA小沟碱基之间。小沟碱基之间。The The nucleosomenucleosomeTh

59、e Histone N-Terminal Tails Stabilize DNA Wrapping around the Octamer两个相邻的小沟槽，在两个两个相邻的小沟槽，在两个DNADNA螺旋之间形成了一个螺旋之间形成了一个仅容一条多肽链的缝隙，四个仅容一条多肽链的缝隙，四个H2BH2B和和H3H3的尾巴从此缝的尾巴从此缝隙中伸出。隙中伸出。而而H4H4和和H2AH2A的尾巴是从的尾巴是从DNADNA螺旋的螺旋的“上面上面”和和“下面下面”伸出。伸出。从从DNADNA螺旋之间或两侧伸出的组蛋白尾巴，其作用类螺旋之间或两侧伸出的组蛋白尾巴，其作用类似于一个螺丝帽上的凹槽，指导似于一个螺丝

60、帽上的凹槽，指导DNADNA以以左手螺旋左手螺旋方式方式缠绕于组蛋白八聚体，即缠绕于组蛋白八聚体，即负超螺旋。负超螺旋。The The nucleosomenucleosomeFigure 7-26 The histone tails emerge from the core of the nucleosome at specific positions, serving as the grooves of a screw to direct the DNA wrapping around the histone core in a left-handed manner.The histone N-terminal tails stabilize DNA wrapping around the octamer 缠绕组蛋白核心的缠绕组蛋白核心的DNA呈负超螺