分子生物学教学课件：8 章 The replication of DNA

1、 第第7章内容回顾章内容回顾1. 染色体、染色质和核小体以及染色体的结构特点。染色体、染色质和核小体以及染色体的结构特点。2. DNA包装成染色体的重要性是什么？包装成染色体的重要性是什么？3. 基因组大小、基因数量和基因密度的概念。基因组大小、基因数量和基因密度的概念。4. 在真核细胞中，影响基因密度大小的因素什么？在真核细胞中，影响基因密度大小的因素什么？5. DNA重复序列的特点和分布。重复序列的特点和分布。6. 染色体中的哪些元件不参与基因的表达？染色体中的哪些元件不参与基因的表达？7. 核小体的组成，组蛋白的结构特点。核小体的组成，组蛋白的结构特点。8. 能够调节染色质结构的因素有哪

2、些？能够调节染色质结构的因素有哪些？ 9. 组蛋白组蛋白N-端尾巴的作用有哪些？端尾巴的作用有哪些？CHAPTER 8:The replication of DNAThe Structure of DNACHAPTER 8CHAPTER 8 In 1957, M. Meselson and F. W. Stahl successfully obtained the experimental proof for the semi-conservative replication of DNA. They did this by inventing a new technique called D

3、ensity Gradient Centrifugation. Their paper was published in 1958 and ever since, the experiment has often been referred to as: one of the most beautiful experiments in biology. How It Began:CHAPTER 8CHAPTER 8Meselson and Stahl Experiment Meselson及及Stahl：提出半保留模型：提出半保留模型M. Meselson and F.W. Stahl. 19

4、58. The replication of DNA in E. coli. Proc. Natl. Acad. Sci. U.S.A. 44: 671-682. semi conservation replicationCHAPTER 8CHAPTER 8Meselson and Stahl.swfCHAPTER 8CHAPTER 8CsCl (氯化铯氯化铯)密度梯密度梯度离心分离度离心分离DNASemi-Conservation Replication视频视频Complications of DNA replication Enzymes The Topological Problem D

5、irection problem: semi-discontinuously PrimingCHAPTER 8CHAPTER 8T1: The Chemistry of DNA SynthesisT2: The Mechanism of DNA PolymeraseT3: The Replication Fork T4: The Specialization of DNA Polymerases T5: DNA Synthesis at the Replication Fork T6: Initiation of DNA ReplicationT7: Binding and Unwinding

6、T8: Finishing ReplicationLecture TopicsGeneralDetailedCHAPTER 8CHAPTER 8The first part describes the basic chemistry of DNA synthesis and the function of the DNA polymerase.CHAPTER 8CHAPTER 8CHAPTER 8CHAPTER 8T1: The Chemistry of DNA DNA synthesis requires deoxynucleoside triphosphates （dNTP） and a

7、primer: template junction. DNA is synthesized by extending the 3 end of the primer. Hydrolysis of pyrophosphate (PPi) is the driving force for DNA synthesis.(1) DNA Synthesis Requires Deoxynucleoside Triphosphates and a Primer:Template junction CHAPTER 8CHAPTER 8Figure 8-1 Substrates required for DN

8、A synthesis.dGTP. dCTP. dATP, or dTTPCHAPTER 8CHAPTER 8(2) DNA Is Synthesized by Extending the 3 End of the PrimerFigure 8-2 Diagram of the mechanism of DNA synthesis.5 3 DNA的结构式的结构式CHAPTER 8CHAPTER 8T2: The Mechanism of DNA Polymerase1. DNA聚合酶以聚合酶以脱氧核苷酸三磷酸脱氧核苷酸三磷酸（dNTPs）为）为底物底物，沿模板的，沿模板的35方方向，将对应的脱

9、氧核苷酸连接到新生向，将对应的脱氧核苷酸连接到新生DNA链的链的3端，端，使新生链沿使新生链沿53方向方向延长延长。DNA Polymerase (Pol)2. 新链与原有的模板链新链与原有的模板链序列序列互补互补，亦与，亦与模板链的原配对链序列一致。模板链的原配对链序列一致。3. DNA聚合酶均以聚合酶均以53方向合成方向合成DNA，且均不能且均不能“重新重新”（de novo）合成）合成DNA，而只能将脱氧核苷酸加到已有的，而只能将脱氧核苷酸加到已有的RNA或或DNA的的3端端羟基羟基上。上。 (1) DNA Pol use a single active site to catalyze

10、 DNA synthesis A single site to catalyze the addition of any of the four dNTPs. Recognition of different dNTP by monitoring the ability of incoming dNTP in forming A-T and G-C base pairs; incorrect base pair dramatically lowers the rate of catalysis. Distinguish between rNTP and dNTP by steric exclu

11、sion of rNTPs from the active site.CHAPTER 8CHAPTER 8CHAPTER 8CHAPTER 8Figure 8-3 Correctly paired bases are required for DNA-Pol-catalyzed nucleotide addition.CHAPTER 8CHAPTER 8Figure 8-4 Schematic illustration of the steric constraints preventing DNA Pol from using rNTP precurson.Distinguish betwe

12、en rNTP and dNTP by steric exclusion of rNTPs from the active site.In DNA Pol, the nucleotide binding pocket is too small to allow the presence of a 2-OH on the incoming nucleotide. This space is occupied by two amino acids (discriminator amino acids辨识氨基酸辨识氨基酸) that make van der Waals contacts with

13、the sugar ring.CHAPTER 8CHAPTER 8(2) DNA Pol Resemble a That Grips the Primer:Template Junction Schematic of DNA Pol bound to a primer: template junctionA similar view of the T7 DNA pol bound to DNAFigure 8-5 Three-dimensional structure of DNA Pol resembles a right hand.CHAPTER 8CHAPTER 8Figure 8-8

14、Illustration of the path of the template DNA through the DNA Pol.ThumbFingersPalmCHAPTER 8CHAPTER 8ThumbFingersPalm维持引物以及活性部位的维持引物以及活性部位的正确位置；正确位置；帮助维持帮助维持DNA聚合酶与其聚合酶与其底物之间的紧密连接。底物之间的紧密连接。一旦引入的一旦引入的dNTP与模板之与模板之间形成正确的碱基配对，手间形成正确的碱基配对，手指域即发生闭合式移动，从指域即发生闭合式移动，从而包围住而包围住dNTP，使引入的，使引入的核苷酸与催化的金属离子密核苷酸与催化的金

15、属离子密切接触，促进催化反应；另切接触，促进催化反应；另外，手指域的移动使模板的外，手指域的移动使模板的磷酸二酯键骨架在活性部位磷酸二酯键骨架在活性部位后立即产生弯曲，使催化位后立即产生弯曲，使催化位点上引物后第一个模板碱基点上引物后第一个模板碱基暴露，便于与下一个配对碱暴露，便于与下一个配对碱基结合。基结合。手掌域不仅可以催化手掌域不仅可以催化dNTP的添加反应，还可以去除错配的的添加反应，还可以去除错配的dNTP；手掌域可以；手掌域可以结合结合2个二价金属离子，从而改变正确碱基配对的个二价金属离子，从而改变正确碱基配对的dNTP和引物和引物3-OH周围的化周围的化学环境；手掌域还负责检查最

16、新加入的核苷酸碱基配对的准确性。学环境；手掌域还负责检查最新加入的核苷酸碱基配对的准确性。(3) DNA Pol are processive enzymes DNA聚合酶是一种延伸酶。聚合酶是一种延伸酶。 1）延伸能力是酶处理多聚体底物时的一种特性。）延伸能力是酶处理多聚体底物时的一种特性。 2）DNA聚合酶的延伸能力定义为每次酶与引物模板接头聚合酶的延伸能力定义为每次酶与引物模板接头结合时所添加核苷酸的平均数结合时所添加核苷酸的平均数。每个。每个DNA聚合酶都有其聚合酶都有其特征性的延伸能力，范围从每次结合时的几个到特征性的延伸能力，范围从每次结合时的几个到5000多多个碱基。个碱基。 D

17、NA合成的速率与聚合酶的延伸能力密切相关。因为，合成的速率与聚合酶的延伸能力密切相关。因为，聚合酶与引物聚合酶与引物-模板接头的最初结合是限速步骤。模板接头的最初结合是限速步骤。 CHAPTER 8CHAPTER 8The thumb helps to maintain a strong association between the DNA polymerase and its substrate.DNA polymerases are processive enzymes; thus the rate of DNA synthesis is dramatically increased (

18、1000 bp/sec).CHAPTER 8CHAPTER 8Figure 8-9聚合酶与引物聚合酶与引物-模板接头的最初模板接头的最初结合是限速步骤（结合是限速步骤（1sec)(4) Exonucleases proofread newly synthesized DNA The occasional flicking of the bases into “wrong” tautomeric form results in incorrect base pair and mis-incorporation of dNTP. (10-5 mistake) The mismatched dNTP

19、 is removed by proofreading exonuclease, a part of the DNA polymerase.How does the exonucleases work? Kinetic selectivityCHAPTER 8CHAPTER 8Figure 8-10CHAPTER 8CHAPTER 8 Proofreading exonucleases removes bases from the 3 end of mismatched DNA. proofreading_function.swfT1: The Chemistry of DNA Synthes

20、isT2: The Mechanism of DNA PolymeraseT3: The Replication Fork T4: The Specialization of DNA Polymerases T5: DNA Synthesis at the Replication Fork T6: Initiation of DNA ReplicationT7: Binding and UnwindingT8: Finishing ReplicationLecture TopicsGeneralDetailedCHAPTER 8CHAPTER 8The second part describe

21、s how the synthesis of DNA occurs in the context of an intact chromosome at replication forks. CHAPTER 8CHAPTER 8T3: The replication fork The junction between the newly separated template strands and the unreplicated duplex DNA. CHAPTER 8CHAPTER 8新分开的模板链与未复制的双链新分开的模板链与未复制的双链DNA之间的连接区称为复制叉。之间的连接区称为复制

22、叉。CHAPTER 8CHAPTER 8(1) Both strands of DNA are synthesized together at the replication fork.Figure 8-11Leading strand(前导链前导链)Lagging strand(后随链后随链)Okazaki fragment(1001000bp)Replication fork(2) DNA helicases unwind the double helix in advance of the replication forkCHAPTER 8CHAPTER 8Figure 8-13DNA解

23、旋酶解旋酶分离双螺旋的两条链。DNA解旋酶具有5-3极性。即在后随链上。CHAPTER 8CHAPTER 8(3) Single-stranded binding proteins (SSBs) stabilize single-stranded DNA Cooperative binding Sequence-independent manner(electrostatic interactions) Figure 8-15单链单链DNA结合蛋白结合蛋白(4) The initiation of a new strand of DNA require an RNA primer Primas

24、e is a specialized RNA polymerase dedicated to making short RNA primers on an ssDNA template. Do not require specific DNA sequence. DNA Pol can extend both RNA and DNA primers annealed to DNA template CHAPTER 8CHAPTER 8(5) RNA primers must be removed to complete DNA replication A joint efforts of RN

25、ase H, DNA Pol & DNA ligase Figure 8-12 Removal of RNA promers from newly synthesized DNA.CHAPTER 8CHAPTER 8CHAPTER 8CHAPTER 8(6) Topoisomerase removes supercoils produced by DNA unwinding at the replication forkFigure 8-16CHAPTER 8CHAPTER 8(7) Replication fork enzymes extend the range of DNA polyme

26、rase DNA Pol can not accomplish replication without the help of other enzymes T4: The Specilization of DNA PolymerasesCHAPTER 8CHAPTER 8 Each organism has a distinct set of different DNA Pols Different organisms have different DNA PolsDNA Pol III holoenzyme: a protein complex responsible for E. coli

27、 genome replicationDNA Pol I: removes RNA primers in E. coli Eukaryotic cells have multiple DNA Pol. Three are essential to duplicate the genome: DNA Pol d d, DNA Pol e e and DNA Pol a a/primase. (What are their functions?) Pol switching in Eukaryotes: the process of replacing DNA Pol a a/primase wi

28、th DNA Pol d d or DNA Pol e e. CHAPTER 8CHAPTER 8CHAPTER 8CHAPTER 8(1) DNA polymerases are specialized for different roles in the cellCHAPTER 8CHAPTER 8DNA Pols of eukaryotes?Figure 8-17 DNA polymerase switching during eukaryotic DNA replicationCHAPTER 8CHAPTER 8 the process of replacing DNA Pola a/

29、primase with DNA Pold d or DNA Pole e. 50100bp10010000bpCHAPTER 8CHAPTER 8(2) Sliding clamps dramatically increase DNA Pol processivity Encircle the newly synthesized double-stranded DNA and the Pol associated with the primer: template junction. Ensures the rapid rebinding of DNA Pol to the same pri

30、mer: template junction, and thus increases the processivity of Pol. Eukaryotic sliding DNA clamp is PCNACHAPTER 8CHAPTER 8Figure 8-18 Structure of a sliding DNA clamp.Figure 8-19 Sliding DNA Clamps Increase the Processivity of associated DNA Pols.CHAPTER 8CHAPTER 8CHAPTER 8CHAPTER 8Figure 8-20 Three

31、-dimensional structure of sliding DNA clamps isolated from different organism.Sliding DNA clamps are found across all organism and share a similar structuretwo copies of b proteintrimer of the PCNA proteintrimer of the gp45 proteinE.coli:EukaryoticT4 phageCHAPTER 8CHAPTER 8(3) Sliding clamps are ope

32、ned and placed on DNA by clamp loaders Clamp loader is a special class of protein complex catalyzes the opening and placement of sliding clamps on the DNA, such a process occurs anytime a primer-template junction is present. Sliding clamps are only removed from the DNA once all the associated enzyme

33、s complete their function.CHAPTER 8CHAPTER 8ATP control of sliding DNA clamp loadingSliding clamp loaderCHAPTER 8CHAPTER 8 At the replication, the leading strand and lagging strand are synthesized simultaneously. To coordinate the replication of both strands, multiple DNA Pols function at the replic

34、ation fork. DNA Pol III holoenzyme is such an example.T5: DNA synthesis at the replication forkCHAPTER 8CHAPTER 8Composition of the DNA Pol III holoenzymeFigure 8-21CHAPTER 8CHAPTER 8Figure 8-22* Trombone modelCHAPTER 8CHAPTER 8CHAPTER 8CHAPTER 8CHAPTER 8CHAPTER 8dna_replication.swf7. RNase H(切除切除RN

35、A引物，引物，DNA Pol I，DNA ligase).(Pol III )Interactions between replication fork proteins form the E. coli replisome Replisome is established by protein-protein interactions1. DNA helicase & DNA Pol III holoenzyme, this interaction is mediated by the clamp loader and stimulates the activity of the helic

36、ase (10-fold)2. DNA helicase & primase, which is relatively week and strongly stimulates the primase function (1000-fold). This interaction is important for regulation the length of Okazaki fragments.CHAPTER 8CHAPTER 8 DNA Pol III holoenzyme, helicase and primase interact with each other to form rep

37、lisome, a finely tuned factory for DNA synthesis with the activity of each protein is highly coordinated.CHAPTER 8CHAPTER 8Interactions between replication fork proteins from the E. coli replisomeCHAPTER 8CHAPTER 8Figure 8-23 Binding of the DNA helicase to DNA pol III holoenzyme stimulates the rate

38、of DNA strand separation.视频视频T1: The Chemistry of DNA SynthesisT2: The Mechanism of DNA PolymeraseT3: The Replication Fork T4: The Specialization of DNA Polymerases T5: DNA Synthesis at the Replication Fork T6: Initiation of DNA ReplicationT7: Binding and UnwindingT8: Finishing ReplicationLecture To

39、picsGeneralDetailedCHAPTER 8CHAPTER 8The third part focuses on the initiation and termination of DNA replication. Note that DNA replication is tightly controlled in all cells and initiation is the step for regulation. CHAPTER 8CHAPTER 8思考下列概念之间的关系：思考下列概念之间的关系： 复制子（复制子（replicon） 复制器（复制器（replicator） 复

40、制起始位点（复制起始位点（ origin of replication ） 起始子（起始子（initiator） 起始位点识别复合体（起始位点识别复合体（origin recognition complex,ORC） 复制体（复制体（replisome）CHAPTER 8CHAPTER 8T6: Initiation of DNA replication(1) Specific genomic DNA sequences direct the initiation of DNA replication Origins of replication, the sites at which DNA

41、 unwinding and initiation of replication occur. Replicator: the entire site of cis-acting DNA sequences sufficient to direct the initiation of DNA replicationInitiator protein: specifically recognizes a DNA element in the replicator and activates the initiation of replicationFigure 8-24 the replicat

42、ion model.CHAPTER 8CHAPTER 8CHAPTER 8CHAPTER 8The identification of origins of replicationBox 8-5 Figure 1 Genetic identification of replicators (origins)CHAPTER 8CHAPTER 8(2) Replicator sequences include initiator binding sites and easily unwound DNAFigure 8-25 Structure of replication.起始子结合位点：绿色起始

43、子结合位点：绿色1）含有起始子蛋白质结合的位点，此位点是组装复制起始机器的核心；2）含有一段富含AT的DNA，此段DNA容易解旋但并不自发进行。在复制器上，DNA的解旋是由复制起始蛋白控制的，这些蛋白被严格调控。如在E.Coli中，复制器为“oriC”。9核苷酸单位的基序是起始子DnaA的结合位点，在oriC”上重复5次，13核苷酸单位的基序重复3次，是起始时单链DNA形成的起始位点。 促进促进DNA解旋的元件：蓝色解旋的元件：蓝色第一段第一段DNA合成的位点：红色合成的位点：红色T7: Binding and Unwinding: origin selection and activatio

44、n by the initiator proteinCHAPTER 8CHAPTER 8 Three different functions of initiator protein: (1) binds to replicator, (2) distorts/unwinds a region of DNA, (3) interacts with and recruits additional replication factors DnaA in E. coli (all 3 functions), origin recognition complex (ORC) in eukaryotes

45、 (functions 1 & 3)Figure 8-26 Functions of the initiator proteins during the initiation of DNA replication.CHAPTER 8CHAPTER 8123CHAPTER 8CHAPTER 8(1) Protein-protein and protein-DNA interactions direct the initiation process DnaA recruits the DNA helicase, DnaB and the helicase loader DnaC DnaB inte

46、racts with primase to initiate RNA primer synthesis, see replisome part for more details.Figure 8-27 Models for E. coli initiation of DNA replicationCHAPTER 8CHAPTER 8Figure 8-27 Models for E. coli initiation of DNA replicationCHAPTER 8CHAPTER 8下列概念之间的关系：下列概念之间的关系：复制子（复制子（replicon）复制器（复制器（replicator

47、）复制起始位点（复制起始位点（ origin of replication ）复制体（复制体（replisome）起始位点识别复合体（起始位点识别复合体（origin recognition complex, ORC）起始子（起始子（initiator）DNAProtein(2) Eukaryotic chromosome are replicated exactly once per cell cycle, which is critical for these organimsCHAPTER 8CHAPTER 81）当）当DNA复制时，必须复制时，必须激活足够多的起始位点，激活足够多的起始

48、位点，以保证每个以保证每个S期中每条染期中每条染色体都被完全复制；色体都被完全复制；2）通常一个细胞周期，起始通常一个细胞周期，起始位点只被激活一次，位点只被激活一次， 即即复制只完成一次。如果某复制只完成一次。如果某些区域没有被复制，就会些区域没有被复制，就会产生染色体断裂。产生染色体断裂。 Figure 8-29 Replicators are inactivated by DNA replicationCHAPTER 8CHAPTER 8(3) Pre-replicative complex (pre-RC) formation directs the initiation of rep

49、lication in eukaryotes Initiation in eukaryotes requires two distinct steps1.Replicator selection: the process of identifying sequences for replication initiation (G1 phase), which is mediated by the formation of pre-RCs at the replicator region. 2.Origin activation:CHAPTER 8CHAPTER 8Figure 8-30 Ste

50、ps in the formation of the prerelicative complex (pre-RC)CHAPTER 8CHAPTER 8AAA+AAA+pre-RC的形成并不导致起始的形成并不导致起始位点位点DNA立即被解旋或者立即被解旋或者DNA聚合酶的募集，而是只聚合酶的募集，而是只有在细胞从细胞周期的有在细胞从细胞周期的G1到到达达S期后，期后，G1期形成的期形成的pre-RC才被激活，并启动复制才被激活，并启动复制起始。起始。 helicase2.Origin activation: pre-RCs are activated by two protein kinases

51、 (Cdk and Ddk) that are active only when the cells enter S phase.CHAPTER 8CHAPTER 8Kinases（激酶）：是一类可以将激酶）：是一类可以将磷酸基团磷酸基团共价连接到靶蛋共价连接到靶蛋白上的蛋白质。白上的蛋白质。所有的激酶在所有的激酶在G1期失活，进入期失活，进入S期后被激活期后被激活。 Assembly of the eukaryotic replication forkCHAPTER 8CHAPTER 8Figure 8-31pre-RC(4) Pre-RC formation and activation

52、is regulated to allow only a single round of replication during each cell cycle.Only one opportunity for pre-RCs to form, and only one opportunity for pre-RC activation.CHAPTER 8CHAPTER 8Cdks: cyclin-dependent kinasesFigure 8-32 Effect of Cdk activity on pre-RC formation and activationCHAPTER 8CHAPT

53、ER 8G1 phaseS/G2/M phaseFigure 8-32 Cell cycle regulation of Cdk activity and pre-RC formatinCHAPTER 8CHAPTER 8Same points between eukaryotic and prokaryotic DNA replication initiationCHAPTER 8CHAPTER 81)Recognize the replicator (DnaA vs ORC ); 2)Assembles the DNA helicase on the replicator (DnaB vs

54、 Mcm2-7); 3) Helicase generates a region of ssDNA for RNA primer synthesis; 4) The replisome assembles and start DNA replication.Different points between eukaryotic and prokaryotic DNA replication initiationCHAPTER 8CHAPTER 8In bacteria cells:1) Initiate replication more than once per cell cycle;2)

55、Focus regulation on the binding of the DnaA initiator protein to the DNA . In eukaryotic cells:1) Initiate replication only once per cell cycle; 2) Focus regulation on the binding of the Mcm helicase to the DNA . T8: FINISHING REPLICATIONCHAPTER 8CHAPTER 8 DNA 复制的完成需要一系列复杂的过程，复制的完成需要一系列复杂的过程，对于环形和线性

56、染色体来说，这些过程有所对于环形和线性染色体来说，这些过程有所不同。不同。 环形染色体的复制叉机器能够复制整个分环形染色体的复制叉机器能够复制整个分子，但是产生的子代分子之间是相互拓扑连子，但是产生的子代分子之间是相互拓扑连接的。接的。 线性染色体最末端处的复制是通过线性染色体最末端处的复制是通过端粒酶端粒酶延伸染色体延伸染色体3端端来解决末端复制的问题。来解决末端复制的问题。(1) Type II topoisomerases are required to separate daughter DNA moleculesCHAPTER 8CHAPTER 8(2) Lagging-strand

57、 synthesis is unable to copy the extreme ends of linear chromosomesCHAPTER 8CHAPTER 8Fig 8-35End replication problemTelomereStructure of human telomereCHAPTER 8CHAPTER 8 端粒是染色体末端的一种特殊结构，是端粒是染色体末端的一种特殊结构，是DNA与相关蛋白质的复合与相关蛋白质的复合体。端粒体。端粒DNA由许多短的富含鸟嘌呤（由许多短的富含鸟嘌呤（G）的重复序列串联而成，）的重复序列串联而成，可长达可长达10kb以上。人的端粒重复

58、序列为以上。人的端粒重复序列为TTAGGG，长达，长达15kb。 端粒主要有两大生理功能：（端粒主要有两大生理功能：（1）维持染色体结构的完整性，防止）维持染色体结构的完整性，防止染色体被核酸酶降解及染色体间相互融和。（染色体被核酸酶降解及染色体间相互融和。（2）防止染色体结构）防止染色体结构基因在复制时丢失，解决了末端复制的难题。基因在复制时丢失，解决了末端复制的难题。 Figure 4-41 Telomeres form a looped structure in the cellCHAPTER 8CHAPTER 8TelomeraseChromosome duplication & se

59、gregationChromosome duplication & segregation端粒的合成主要依靠端粒的合成主要依靠端粒酶端粒酶来催化。端粒酶是来催化。端粒酶是RNA与蛋白质组成与蛋白质组成的核糖核蛋白，是一种的核糖核蛋白，是一种RNA依赖性依赖性DNA聚合酶聚合酶。人类端粒酶。人类端粒酶RNA成分已被成功克隆，它包括与端粒重复序列互补的成分已被成功克隆，它包括与端粒重复序列互补的11个核苷个核苷酸酸5-CUAACCCUAAC-3。端粒酶的主要作用是维持端粒的长度。它能利用端粒端粒酶的主要作用是维持端粒的长度。它能利用端粒3端单链为端单链为引物，自身的引物，自身的RNA为模板合成端粒

60、重复序列添加到染色体末端，为模板合成端粒重复序列添加到染色体末端，从而延长端粒的长度。人的生殖细胞、造血干细胞及从而延长端粒的长度。人的生殖细胞、造血干细胞及T、B淋巴淋巴细胞中端粒酶有不同程度的表达，细胞中端粒酶有不同程度的表达，而在正常的体细胞中，端粒酶而在正常的体细胞中，端粒酶处于失活状态，因此体细胞随细胞分裂次数的增加端粒逐渐缩短。处于失活状态，因此体细胞随细胞分裂次数的增加端粒逐渐缩短。端粒的长度与有丝分裂次数相关，所以端粒又有细胞的端粒的长度与有丝分裂次数相关，所以端粒又有细胞的“有丝分有丝分裂钟裂钟”之称之称. telomerase_function.swfTelomerase