分子生物學(xué)教學(xué)課件：CHAPTER 7 Chromosomes, chromatin,

1、回顧上一節(jié)的內(nèi)容回顧上一節(jié)的內(nèi)容1. 什么是分子生物學(xué)？什么是分子生物學(xué)？2. 分子生物學(xué)發(fā)展的三個(gè)階段：分子生物學(xué)發(fā)展的三個(gè)階段： 1）準(zhǔn)備和醞釀階段：）準(zhǔn)備和醞釀階段：產(chǎn)生了兩點(diǎn)對(duì)生命本質(zhì)的認(rèn)識(shí)上的重大突破。 證明DNA是遺傳物質(zhì)的兩個(gè)關(guān)鍵性實(shí)驗(yàn)：肺炎球菌轉(zhuǎn)化實(shí)驗(yàn)和T4 噬菌體在大腸桿菌的實(shí)驗(yàn)。 2）現(xiàn)代分子生物學(xué)的誕生和發(fā)展：）現(xiàn)代分子生物學(xué)的誕生和發(fā)展：DNA雙螺旋結(jié)構(gòu)的提出。雙螺旋結(jié)構(gòu)的提出。 逐漸完善了對(duì)DNA復(fù)制機(jī)理的認(rèn)識(shí)；逐步闡明了RNA轉(zhuǎn)錄合成的 機(jī)理；認(rèn)識(shí)了蛋白質(zhì)翻譯合成的基本過程；補(bǔ)充和完善了遺傳信 息傳遞的中心法則；對(duì)蛋白質(zhì)結(jié)構(gòu)與功能的進(jìn)一步認(rèn)識(shí)。 3）70年代后期以基

2、因工程技術(shù)的出現(xiàn)作為新的里程碑，標(biāo)志著人類年代后期以基因工程技術(shù)的出現(xiàn)作為新的里程碑，標(biāo)志著人類 深入認(rèn)識(shí)生命本質(zhì)并能改造生命的新的時(shí)期的開始。深入認(rèn)識(shí)生命本質(zhì)并能改造生命的新的時(shí)期的開始。 DNA重組技術(shù)；基因組研究的發(fā)展；單克隆抗體和遺傳工程單克隆抗體 的產(chǎn)生；基因表達(dá)調(diào)控機(jī)理的逐步完善和細(xì)胞信號(hào)傳導(dǎo)機(jī)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: 細(xì)胞核細(xì)胞核; Nucleolus: 核仁核仁Nucleoid: 類核類核Mitosis: 有絲分裂；有絲分裂； Meiosis：減數(shù)分裂：減數(shù)分裂Interphase：分裂間期：分裂間期Histone: 組蛋白；組蛋白； Nucleosome: 核小體核小體Chromosome: 染色體；染色體；Chromatin: 染色質(zhì)；染色質(zhì)；eu-; hetero-Centromere (著絲粒）

5、；著絲粒）； Telomere（端粒）（端粒）Repetitive DNA (重復(fù)重復(fù)DNA)Tandem gene cluster（串聯(lián)基因簇）（串聯(lián)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 染色體是細(xì)胞內(nèi)具有遺傳性質(zhì)的物體，易被堿性染料染成深色而得其名，是由染色體是細(xì)胞內(nèi)具有遺傳性質(zhì)的物體，易被堿性染料染成深色而得其名，是由DNA、蛋白質(zhì)和少量、蛋白質(zhì)和少量RNA組成的線狀或棒狀物，在細(xì)胞分裂時(shí)，染色質(zhì)絲經(jīng)過組成的線狀或棒狀物，在細(xì)胞分裂時(shí)，染色質(zhì)絲經(jīng)過螺旋化、折疊、包裝成為染色體，為顯微鏡下可見的具有不同形狀的小體。螺旋化、折疊、包裝成為染色體，為顯微鏡下可見的具有不同形狀的小體。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染色體是由線性雙鏈染色體是由線性雙鏈DNA分子同分子同蛋白質(zhì)蛋白質(zhì)形成的復(fù)合物。形成的復(fù)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（基因組序列和染色體多樣性）（基因組序列和染色體多樣性）2. The chromosome duplication and segregation（染色體的復(fù)制和分離）（染色體的復(fù)制和分離）3. The Nucleosome（核小體）（核小體）4. Higher-order chromatin structure5. Regulation of chromatin structure6. Nucleosome assem

11、bly（核小體的組裝）（核小體的組裝）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單倍單倍體體雙倍體細(xì)胞雙倍體細(xì)胞單倍體細(xì)胞單倍體細(xì)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同源染色體同源染色體 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 基因組（基因組（Genome）：指單倍體細(xì)胞中包括編碼序列和非編）：指單倍體細(xì)胞中包括編碼序列和非編碼序列在內(nèi)的全部碼序列在內(nèi)的全部DNA分子分子。更確切的說，。更確切的說，基因組基因組是單倍體是單倍體細(xì)胞核細(xì)胞核內(nèi)的全部內(nèi)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生物的復(fù)雜度越高所需的基因也越多。一般原核細(xì)胞的基因組不足生物的復(fù)雜度越高所需的基因也越多。一般原核細(xì)胞的基因組不足10Mb，而單，而單細(xì)胞真核生物的基因組都小于細(xì)胞真核生物的基因組都小于50Mb，多核細(xì)胞的基因組甚至超過，多核細(xì)胞的基因組甚至超過10萬萬Mb。 生物的復(fù)雜性與基因組的大小并不完全成比例增生物的復(fù)雜性與基因組的大小

16、并不完全成比例增加。如果蠅和蝗蟲，雖然是具有相似復(fù)雜度的生物加。如果蠅和蝗蟲，雖然是具有相似復(fù)雜度的生物體，但是基因組大小明顯不同。體，但是基因組大小明顯不同。 基因組測(cè)序的結(jié)果發(fā)現(xiàn)，越是復(fù)雜的生物體，參基因組測(cè)序的結(jié)果發(fā)現(xiàn)，越是復(fù)雜的生物體，參與編碼的基因序列在整個(gè)基因組序列中所占的比例與編碼的基因序列在整個(gè)基因組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。 基因組和基因的數(shù)量，隨著生物體的復(fù)雜而增多，基因組和基因的數(shù)量，隨著生物體的復(fù)雜而增多，但是基因的密度卻降低。但是基因的密度卻降低。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.基因組密度的簡單衡量方法是每基因組密度的簡單衡量方法是每Mb基因組基因組DNA上基因的平均數(shù)目。上基因的平均數(shù)目。 Two factors contribute to the decreased gen

18、e density in eucaryotic cells：1. Increases in gene size: 非連續(xù)性的蛋白質(zhì)編碼區(qū)被非連續(xù)性的蛋白質(zhì)編碼區(qū)被內(nèi)含子內(nèi)含子 (introns) 所分開。內(nèi)含子的存在極大地增加了編碼所分開。內(nèi)含子的存在極大地增加了編碼一個(gè)基因所需的一個(gè)基因所需的DNA的長度。的長度。.2. Increases in the DNA between genes（intergenic sequences，基因間序列基因間序列）: 單一的和重單一的和重復(fù)的，這些序列參與指導(dǎo)和調(diào)控復(fù)的，這些序列參與指導(dǎo)和調(diào)控DNA的的轉(zhuǎn)錄轉(zhuǎn)錄（regulatory sequences

19、，調(diào)控序列）調(diào)控序列）。隨著生物體復(fù)雜度的提高，編。隨著生物體復(fù)雜度的提高，編碼更多的基因，調(diào)控基因表達(dá)的調(diào)控序列也更加復(fù)雜，其序碼更多的基因，調(diào)控基因表達(dá)的調(diào)控序列也更加復(fù)雜，其序列大小也不斷增加。列大小也不斷增加。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 衛(wèi)星衛(wèi)星DNA(satelliteDNA)：是一類高度重復(fù)：是一類高度重復(fù)序列序列DNA在介質(zhì)氯化銫中作在介質(zhì)氯化銫中作密度密度梯度離心后，梯度離心后，在離心管內(nèi)按其大小分布形成了不同的條帶；在離心管內(nèi)按其大小分布形成了不同的條帶；根據(jù)熒光強(qiáng)度的分析，可以看到在一條主帶根據(jù)熒光強(qiáng)度的分析，可以看到在一條主帶以外還有一個(gè)或多個(gè)小的衛(wèi)

22、星帶。以外還有一個(gè)或多個(gè)小的衛(wèi)星帶。Chromosome sequence and diversityChromosome sequence and diversityCsCl 密度梯度離心分離密度梯度離心分離DNA串聯(lián)重復(fù)的串聯(lián)重復(fù)的DNA分散重復(fù)的分散重復(fù)的DNASatellite DNAChromosome sequence and diversityChromosome sequence and diversity衛(wèi)星衛(wèi)星DNA(satelliteDNA)：GC含量一般少于主帶中的含量一般少于主帶中的DNA，浮力密度也低。，浮力密度也低。按按其重復(fù)單元的核苷酸的多少可分為兩類：一類是其

23、重復(fù)單元的核苷酸的多少可分為兩類：一類是小衛(wèi)星小衛(wèi)星DNA，由幾百個(gè)核苷酸，由幾百個(gè)核苷酸對(duì)的單元重復(fù)組成；另一類是對(duì)的單元重復(fù)組成；另一類是微衛(wèi)星微衛(wèi)星DNA，由，由2個(gè)到個(gè)到20個(gè)左右的核苷酸對(duì)的單元個(gè)左右的核苷酸對(duì)的單元重復(fù)成百上千次所組成。關(guān)于衛(wèi)星的產(chǎn)生機(jī)理，認(rèn)為可能是重復(fù)成百上千次所組成。關(guān)于衛(wèi)星的產(chǎn)生機(jī)理，認(rèn)為可能是DNA復(fù)制和修復(fù)過復(fù)制和修復(fù)過程中的滑動(dòng)錯(cuò)配或染色體減數(shù)分裂時(shí)姊妹染色單體不均程中的滑動(dòng)錯(cuò)配或染色體減數(shù)分裂時(shí)姊妹染色單體不均等交換的結(jié)果。等交換的結(jié)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. 核心序列為核心序列為25bp 25bp ，主要存在于染色體靠近端粒處，主要存在于染色體靠近端粒處, ,與與DNADNA復(fù)制有關(guān)。染色體其他位置的小衛(wèi)星復(fù)制有關(guān)。染色體其他位置的小衛(wèi)星DNA,DNA,功能不清。功能不清。Chromosome sequence and diversityChromosom

25、e sequence and diversityMicrosatellite DNA (微衛(wèi)星微衛(wèi)星DNA)指以指以2 27 7個(gè)堿基為核心單位串聯(lián)重復(fù)而成的個(gè)堿基為核心單位串聯(lián)重復(fù)而成的一類序列，又稱為一類序列，又稱為STR short tandem STR short tandem repeat(repeat(短串聯(lián)重復(fù)序列）短串聯(lián)重復(fù)序列）Chromosome sequence and diversityChromosome sequence and diversityA class of miniaturized satellite 微衛(wèi)星在基因組中的功能尚不清楚，可能參與遺傳物微衛(wèi)星

26、在基因組中的功能尚不清楚，可能參與遺傳物質(zhì)的結(jié)構(gòu)改變，基因調(diào)控及細(xì)胞分化等過程，有自身質(zhì)的結(jié)構(gòu)改變，基因調(diào)控及細(xì)胞分化等過程，有自身特異結(jié)合蛋白，尚能直接編碼蛋白質(zhì)。如脆性特異結(jié)合蛋白，尚能直接編碼蛋白質(zhì)。如脆性X綜合綜合癥基因內(nèi)癥基因內(nèi)(CGG)n串聯(lián)重復(fù)，編碼串聯(lián)重復(fù)，編碼30個(gè)精氨酸；個(gè)精氨酸；(CAG)n是人類基因庫是人類基因庫DNA序列外顯子中發(fā)現(xiàn)最多的序列外顯子中發(fā)現(xiàn)最多的一種。另外，微衛(wèi)星在促進(jìn)染色體凝集，維持染色體一種。另外，微衛(wèi)星在促進(jìn)染色體凝集，維持染色體結(jié)構(gòu)等方面也有作用，可參與染色體折疊、染色體端結(jié)構(gòu)等方面也有作用，可參與染色體折疊、染色體端粒形成等。粒形成等。(CA/

27、GT)n、(GATA)n可能與致育性、性可能與致育性、性別分化、別分化、X染色體失活有關(guān)。染色體失活有關(guān)。 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復(fù)制起始位點(diǎn)是復(fù)制起始位點(diǎn)是DNA復(fù)制機(jī)器組裝和復(fù)制起始的位點(diǎn)。復(fù)制機(jī)器組裝和復(fù)制起始的位點(diǎn)。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；真核細(xì)真核細(xì)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.中著絲粒中著絲粒亞中著絲粒亞中著絲粒亞端部著絲粒亞端部著絲粒端

33、部著絲粒端部著絲粒CentromeresCentromere (著絲粒著絲粒): a region of chromosome where two sister chromatids are joined at metaphase.Mitotic chromosome at metaphaseKinetochore動(dòng)粒動(dòng)粒Centromere著絲粒著絲粒 Kinetochore (動(dòng)粒動(dòng)粒): a protein complex attaching microtubules(微管微管) of mitotic spindle (紡錘體紡錘體).Mitotic spindle（有絲分裂（有絲分裂紡

34、錘體）紡錘體）Telomere 端粒端粒 Chromosome duplication & segregationChromosome duplication & segregationKinetochore (動(dòng)粒動(dòng)粒):由多種由多種蛋白質(zhì)在有絲分裂染色體著絲蛋白質(zhì)在有絲分裂染色體著絲粒部位形成的一種圓盤狀結(jié)構(gòu)。粒部位形成的一種圓盤狀結(jié)構(gòu)。微管與之連接，與染色體分離微管與之連接，與染色體分離密切相關(guān)。每一個(gè)中期染色體密切相關(guān)。每一個(gè)中期染色體含有兩個(gè)動(dòng)粒，位于著絲粒的含有兩個(gè)動(dòng)粒，位于著絲粒的兩側(cè)。兩側(cè)。 間期間期前期前期中期中期后期后期末期末期胞質(zhì)分裂胞質(zhì)分裂Centrom

35、eres direct the formation of an elaborate protein complex, in this case, called a kinetochore.動(dòng)粒的作用機(jī)制是著絲粒動(dòng)粒的作用機(jī)制是著絲粒DNA與蛋白質(zhì)纖維（微管）相互作與蛋白質(zhì)纖維（微管）相互作用，拉動(dòng)姐妹染色體相互遠(yuǎn)離用，拉動(dòng)姐妹染色體相互遠(yuǎn)離并進(jìn)入兩個(gè)子細(xì)胞并進(jìn)入兩個(gè)子細(xì)胞. Kinetochore (動(dòng)粒動(dòng)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)端粒是染色體末端的DNA重復(fù)序列，作用是保持染色體的完整性。細(xì)胞分裂一次，由于DNA復(fù)制時(shí)的方向必須從5方向到3方向，DNA每次復(fù)制端粒就縮短一點(diǎn)（參見岡崎片段）。一旦端粒消耗殆盡，染色體則易于突變而導(dǎo)致動(dòng)脈硬化和某些癌癥。因

37、此，端粒和細(xì)胞老化有明顯的關(guān)系。一個(gè)基因組內(nèi)的所有端粒都是由相同的重復(fù)序列組成，但不同物種的端粒的重復(fù)序列是不同的。Function of TelomeresChromosome duplication & segregationChromosome duplication & segregation保護(hù)染色體不被核酸酶降解；保護(hù)染色體不被核酸酶降解；防止染色體相互融合；防止染色體相互融合；為端粒酶提供底物，解決為端粒酶提供底物，解決DNA復(fù)制的末復(fù)制的末端隱縮，保證染色體的完全復(fù)制。端隱縮，保證染色體的完全復(fù)制。端粒、著絲粒和復(fù)制原點(diǎn)是染色體保持端粒、著絲粒和復(fù)制原點(diǎn)是染色體

38、保持完整和穩(wěn)定的三大要素。完整和穩(wěn)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細(xì)胞完成一輪分裂所需要的過程叫細(xì)胞周期。大多數(shù)真核細(xì)胞的分裂會(huì)使子細(xì)胞細(xì)胞完成一輪分裂所需要的過程叫細(xì)胞周期。大多數(shù)真核細(xì)胞的分裂會(huì)使子細(xì)胞維持與母細(xì)胞一致的染色體數(shù)目叫細(xì)胞的有絲分裂。維

40、持與母細(xì)胞一致的染色體數(shù)目叫細(xì)胞的有絲分裂。Figure 7-11 The events of S phasechromatidCohesion(黏粒黏粒)：復(fù)制后的姐妹染色單體通過黏粒的分子聚復(fù)制后的姐妹染色單體通過黏粒的分子聚在一起，即姐妹染色單體的附著。這種狀態(tài)一直維持到染色單體的在一起，即姐妹染色單體的附著。這種狀態(tài)一直維持到染色單體的相互分離。相互分離。03a_function_cohesin.swfFigure 7-12 The events of M phaseMitotic spindle染色單體的配對(duì)由組裝在每個(gè)著絲粒處的動(dòng)粒所介導(dǎo)。黏粒被水解后，抵抗有絲染色單體的配對(duì)由組裝

41、在每個(gè)著絲粒處的動(dòng)粒所介導(dǎo)。黏粒被水解后，抵抗有絲分裂紡錘體的拉力消失，姐妹染色單體被迅速地拉向有絲分裂紡錘體相反的兩極。分裂紡錘體的拉力消失，姐妹染色單體被迅速地拉向有絲分裂紡錘體相反的兩極。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有絲分裂維持親本有絲分裂維持親本染色體的數(shù)目；真染色體的數(shù)目；真核細(xì)胞的另一種類核細(xì)胞的另一種類型是減數(shù)分裂。與型是減數(shù)分裂。與有絲分裂不同的是有絲分裂不同的是進(jìn)入減數(shù)分裂的細(xì)進(jìn)入減數(shù)分裂的細(xì)胞必需是二倍體，胞必需是二倍體，染色體進(jìn)行兩輪分染色體進(jìn)行兩輪分離，最終和母細(xì)胞離，最終和母細(xì)胞相比，產(chǎn)生了半數(shù)相比，產(chǎn)生了半數(shù)的

44、染色體。的染色體。間期間期前期前期中期中期后期后期末期末期胞質(zhì)分裂胞質(zhì)分裂減數(shù)分裂間期減數(shù)分裂間期減數(shù)分裂減數(shù)分裂I期期減數(shù)分裂減數(shù)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. 細(xì)胞周期的間期有兩個(gè)作用：一是為下一個(gè)細(xì)細(xì)胞周期的間期有兩個(gè)作用：一是為下一個(gè)細(xì)胞周期階段作好時(shí)間上的準(zhǔn)備，二是檢查上一胞周期階段作好時(shí)間上的準(zhǔn)備，二是檢

45、查上一個(gè)細(xì)胞周期階段是否正確完成。如果在細(xì)胞周個(gè)細(xì)胞周期階段是否正確完成。如果在細(xì)胞周期的前一階段存在問題，細(xì)胞周期檢查點(diǎn)期的前一階段存在問題，細(xì)胞周期檢查點(diǎn)(cell cycle checkpoint)則停止細(xì)胞周期并給細(xì)胞提則停止細(xì)胞周期并給細(xì)胞提供時(shí)間以完成這一步。供時(shí)間以完成這一步。 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是一種內(nèi)切核酸酶，是一種內(nèi)切核酸酶，作用于單鏈及雙鏈核酸，生成作用于單鏈及雙鏈核酸，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. 任何細(xì)胞中都含有沒有包裝成的長任何細(xì)胞中都含有沒有包裝成的長DNA，雖然這些，雖然這些DNA沒有直接沒有直接與核小體結(jié)合，但是可以結(jié)合非組蛋白，從而參與調(diào)控基因的表達(dá)、與核小體結(jié)合，但是可以結(jié)合非組蛋白，從而參與調(diào)控基因的表達(dá)、復(fù)制或重組功能等。復(fù)制或重組功能等。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的小溝面對(duì)組蛋白八聚體時(shí)可發(fā)現(xiàn)一個(gè)接觸的小溝面對(duì)組蛋白八聚體時(shí)可發(fā)現(xiàn)一個(gè)接觸位點(diǎn)，共發(fā)現(xiàn)位點(diǎn)，共發(fā)現(xiàn)1414個(gè)不同的位點(diǎn)。個(gè)不同的位點(diǎn)。組蛋白與組蛋白與DNADNA之間是通過大量的氫鍵（約之間是通過大量的氫鍵（約4040個(gè)）所介個(gè)）所介導(dǎo)的。導(dǎo)的。這些氫鍵大多位于蛋白質(zhì)和這些氫鍵大多位于蛋白質(zhì)和DNADNA小溝附近磷酸二酯骨小溝附近磷酸二酯骨架上的氧原子之間。架上的氧原子之間。只有只有7 7個(gè)氫鍵位于蛋白質(zhì)側(cè)鏈和個(gè)氫鍵位于蛋白質(zhì)側(cè)鏈和DNADNA小溝堿基之間。小溝堿基之間。The The nucleosomenucleosomeTh

59、e Histone N-Terminal Tails Stabilize DNA Wrapping around the Octamer兩個(gè)相鄰的小溝槽，在兩個(gè)兩個(gè)相鄰的小溝槽，在兩個(gè)DNADNA螺旋之間形成了一個(gè)螺旋之間形成了一個(gè)僅容一條多肽鏈的縫隙，四個(gè)僅容一條多肽鏈的縫隙，四個(gè)H2BH2B和和H3H3的尾巴從此縫的尾巴從此縫隙中伸出。隙中伸出。而而H4H4和和H2AH2A的尾巴是從的尾巴是從DNADNA螺旋的螺旋的“上面上面”和和“下面下面”伸出。伸出。從從DNADNA螺旋之間或兩側(cè)伸出的組蛋白尾巴，其作用類螺旋之間或兩側(cè)伸出的組蛋白尾巴，其作用類似于一個(gè)螺絲帽上的凹槽，指導(dǎo)似于一個(gè)螺絲

60、帽上的凹槽，指導(dǎo)DNADNA以以左手螺旋左手螺旋方式方式纏繞于組蛋白八聚體，即纏繞于組蛋白八聚體，即負(fù)超螺旋。負(fù)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 纏繞組蛋白核心的纏繞組蛋白核心的DNA呈負(fù)超螺