第5章DNA损伤和修复

1、Chapter 5 DNA damage and repair1主要内容1) 诱变2) DNA损伤3) DNA修复2教学要求：1)熟悉DNA损伤的原因、类型2)掌握DNA修复的方式5.1 Mutagenesis5.2 DNA damage5.3 DNA Repair5.1 Mutagenesis （诱变）? 5.1.1 The reasons of mutation? 5.1.2 Types of mutations? 5.1.3 Mutagens （诱变剂 ）? 5.1.4 mutagenesis 诱变（）5.1 Mutagenesis （诱变）? Mutation （ 突变） = Perm

2、anent, heritable alterations in the base sequence of a DNA molecule（是DNA碱基序列水平上永久性的、可遗传的变化）5.1.1 The reasons of mutation? Spontaneous errors自发性错误）:in DNA replication or meiotic recombination? Mutagen （诱变物）: A consequence of the damaging effects of physical or chemicalmutagens on DNA? Mutagen= mutati

3、on causing agent? Essentially all mutagens are carcinogen敛癌物）? Most carcinogens are mutagens5.1.2 Types of mutations? Multisite（ 多位点）：- Cause gross（ 重的）chromosome abnormalities（奇形）；-Involve large regions of DNA ；-Arise during meiosis（在减数分裂期发生）? Point mutations （点突变）-Involve only one or afew nucleoti

4、des；- Arise during DNA replication- Require two errors ： 1） An error during DNA replication ； 2） Failure to correct that errorTypes of point mutationsInversions:ACBDEFDuplications ABCDEEFDeletions:ABCD-FInsertions:ABCDSEFSubstitutions: ATCDEFTypes of point mutationsThe phenotypic effect of point mut

5、ation（ 点突变的表型效应）? Missense mutation（昔义突变）：碱基序列的改变引起了产物氨基酸序列的改变。? Nonsense mutation （无义突变）：某个碱基的改变使代表某种氨基酸的密码子变为蛋白质合成的终止密码子（ stop codon，TAG, TAA, TGA） 。? Samesensemutation （同义突变）：是指未改变产物氨基酸序列的密码子变化。Noeffect （silent mutation沉默突变）Reading frame （阅读框）is one of the three possible ways in which an mRNA seq

6、uenceof nucleotides can be read as a series of base triplets三个一组 to specify the amino acids in a protein chain.ORF （open reading framed放性I阅读框）:from start codon to stop codonWhat is the first defense against mutations?On the one hand, the actual error rate of the polymerase, before editing, is of the

7、 order of 0.1% to 1.0%.However, the overall error rate for DNA replication is 1 error in 109 to 1010 base pairs.This phenomenal fidelity is achieved in three ways.Replication fidelity （复制的保真度）First, Watson-Crick base pairingtoSecond, DNA polymerases have the ability to edit （"proofread"） t

8、heir work （3 exonuclease activity of the polymerases）.Third, post-replication repair of DNA （mismatch repair）.5.1.3 Mutagens 例变剂）Physical mutagens （物理诱变齐J ）? High-energy ionizing radiation（电离辐射）： X-rays and -raysstrand breaks 断链）， base/sugar destruction （碱基 /核糖损伤）? Nonionizing radiation （非电离辐射）：UV l

9、ight pyrimidine dimers （示呢二聚 体）Chemical mutagens （化学诱变剂）? Base analogs（碱基类彳以物）:mispair, direct mutagenesis? Nitrous acid （亚硝酸）：deaminates C to produce U （脱氨基作用）? Alkylating agents （烷化剂）? Arylating agents （芳基化剂）5.1.4 Mutagenesis 例变）? Direct mutagenesis直接诱变）： results from the presence of stable, unrep

10、aired base with altered base pairing properties in the DNA.Indirect mutagenesis何接诱变）:The mutation is introduced as a result of an error-prone repair （倾向差错的修复） .一Translesion的越损伤）DNA synthesis : Insertion of bases opposite the unrepaired lesion regardless of the original sequence不顾原始序列如何，在未修复的损伤 序列对面插

11、入碱基）-to maintain the DNA integrity but not the sequenceaccuracy （只彳於证DNA序列的完整性，不保证序列的精确性）5.2 DNA damageDNA lesions损伤：oxidative damages 化性损伤Alkylation 烷基化bulky adducts聚化力口合物5.2.1 DNA lesions （DNA 损伤）? DNA lesions （DNA 损伤）： An alteration to the normal chemical or physical structure of the DNA （DNA正常的化

12、学或物理结构的改变）5.2.3 Alkylation （烷基化作用）5.2.4 Bulky adducts （聚化加合物）Bulky lesions such as pyrimidine dimers and arylating 芳基化 agent adducts distort the double helix and cause localized 局部 denaturation. This disrupts the normal functioning of the DNA.口密噬二聚体的形成喀呢二聚体是如何造成DNA损伤的？?由于相邻的胸腺喀呢产生二聚体，两个碱基平面被环丁基所扭转，引

13、起双螺旋构型的局部变化，同时氢键结合力也显著减弱。这样，当有胸腺喀呢二聚体的DNA作为模板进行复制时，Polin将两个腺喋吟核甘酸加上去，但由于不能很好地形成 氢键，然后，又由3'-5'校对功能而将之水解。如此反复发生，而产生了一个空 耗的过程，即大量的dATP被分解，而DNA复制却毫无进展。喀呢二聚体是如何造成DNA损伤的？?由于蛋白质仍在不断合成，而DNA不能复制，细胞也就不能分裂，结果出现细丝 状的所谓蛇形细胞，最后导致细胞死亡。5.3 DNA Repair （ DNA 修复）? Direct Repair：-Photoreactivation （光复活）-Alkyltr

14、ansferase （烷基转移酶）? Exision repair （切除修复）? Mismatch repair （错配彳复）? Post-Replication Repair （复制后修复）-recombinational repair （重组修复）-SOS repair5.3.1 Photoreactivation 洸复活）在可见光的存在下，DNA光解酶（photolyase,光复活酶）可将 环丁烷二聚体再 分解为单体。这些酶含有可吸收蓝光并将能量转移到待切环丁烷环中的辅基。E. coli的光解酶含有2个色素分子，N5, N10-次甲基四氢叶酸和还原性的黄素腺喋吟二核甘酸（ FAD）。光

15、 复活对嘧啶二聚体是专一性的。是损伤被 “直接修复 ”的一种例子，是无差错的。Photoreactivation of Thymine Dimers? Processis catalyzed®化 in a process similar to photosynthesis- harvesting energy from light? Human cells do not contain photolyase5.3.2 Alkyltransferase （烷基转移酶）?alkylation of dna bases:?Can blockffl碍 DNA replication beca

16、use of modified bases that are formed?Sometime used in chemotherap y（化学疗法）to block cell division?Usually purines are altered- spectrums谱 of products variesMost highly mutagenic of these products:GuanineO6 -alkyl guanine错配的O6-methylguanin琲口thymine一起造成 GC替换为 AT。无差错直接修复?烷基转移酶特异性地转移O6耳基鸟口S吟或O6 -乙基鸟喋吟上的甲

17、基或乙基基 团到酶分子的半胱氨酸上，从而保护 DNA免受烷基化诱变。5.3.3 Exision repair （切除修复）? including一Nucleotide Excision Repair （ NER,核甘酸切除修复） E. coli UvrABC endonuclease系统一Base excision repair （BER 碱基切除修复）- Uracil-DNA N glycosylase system（糖基化酶系统）? Is a ubiquitous mechanism repairing a variety of lesions 是修复多种损伤的普遍性机制?Error-fr

18、ee repai吮差错修复核苷酸切除修复 Excision repair systems inE. coli1 .uvrA 和 uvrB 蛋白识别损伤部位。2 .发现二聚体后，uvrA解离，uvrC加入。3 .uvrB和uvrC复合体在损伤部位的两端造成单链缺口。4 .损伤部位的DNA被DNA螺旋酶（uvrD）移走。5 . DNA上的缺口由DNA聚合酶I和连接酶填补）。核苷酸切除修复 Excision repair systems in eukaryotesNucleotide excision repairxeroderma pigmentosum （XP,着色性干皮病）是一种常染色体隐性紊

19、乱，在表型上表现为对阳光极为敏感，极易产生皮肤癌。XP患者缺乏对包括由紫外线引起的大块 DNA损伤的核甘酸切除功能，至少有7种不同 基因的缺陷可导致XP。Base excision repair （BER, 碱基切除修复）5.3.4 Mismatch repair （错配修复）? 用于修复在复制中错配并漏过校正检验的任何碱基。可使复制的保真性提高102-103倍。? occurs just after replication? must distinguish the parent from the daughter strand5.3.5 Recombinational Repair（重组修

20、复）? Present in prokaryotic and eukaryotic cells? Only poorly understood? We know it exists because UvrA- and RecA- cells are much more sensitive to UV than cells containing only one mutation.Recombinational Repair? 子链在母链损伤处形成缺口? Depends on RecA protein important for recombination and repair; it cata

21、lyzes strand pairing.? 以子链为模板填补母链的缺口? 复制重新开始，损伤位点被切除修复5.3.6 SOS repair（ SOS 修复）? UV reactivation （紫外激活反应）或 W reactivation （ W激活反应）：-50年代J. Weigle发现，用经紫外线照射后的噬菌体感染用低剂量 UV照射过的大 肠杆菌时，噬菌体的存活率比感染未用低剂量UV照射的大肠杆菌明显增加，突变率也随之增加，这一效应称为UV reactivation （紫外激活反应）或W reactivation（W激活反应）。? the increased survival in t

22、he uv irradiated host is due to the induction of the SOS-repair system in the hos t.（存活率上升是由于紫外线照射引起了寄主 SOS修复 系统的感应）? SOSt复是一种旁路系统,它允许新生的DNA链越过胸腺喀呢二聚体而生长，其 代价是保真度的极大降低，这是一种易错修复系统。SOS反应机制? Metabolic system that helps the cell survive in periods of potentially lethal stresses潜在的致死压力下，细胞的新陈代谢系统帮助细胞存活）-

23、Induced by （诱导因素）：? UV irradiation （紫外照射）? Thymine starvation （胸腺喀呢饥饿）? Treatment with DNA modifying enzymes（ DNA 修饰酶处理）? Inactivation of genes essential to replication （DNA 复制必须的基因失活）The SOS genes SOS repairDiseases caused by defects in RecQ helicase genesWerner syndrome? premature agingi早衰老,beginning in 20' s; life exp. 45? Predisposition患病白体质 to malignanciesDiseases caused by defect