1、DNA damage, repair and recombination DNA损伤、修复和重组,Chapter 6,Damage and Mutation （损伤和突变）,DNA damage is simply a chemical alteration to DNA. DNA损伤仅是DNA分子的化学改变 A mutation is a heritable permanent change in the base sequence of DNA. DNA碱基序列水平上的永久性可遗传的改变。,Unrepaired DNA damage can lead to mutation. 未修复的DN
2、A损伤可导致突变,The nature of mutations (突变的本质),Point mutation 点突变 （Substitution 碱基置换）Mutations that alter a single nucleotide.,Transition (转换): A G T C,Transversion (颠换): A T or C T A or GG T or C C A or G,Frameshift mutation 移码突变Insertion 插入 Deletion 缺失,The examples of mutations 突变的例子,-T-C-G-G-C-T-G-T-A-
3、C-G-A-G-C-C-G-A-C-A-T-G-C-,transition转换,-T-C-G-A-G-C-T-G-T-A-C-G-A-G-C-T-C-G-A-C-A-T-G-C-,Insertion 插入,A,-T-C-G-C-T-G-T-A-C-G-A-G-C-G-A-C-A-T-G-C-,Deletion 缺失,T,-T-C-G-T-C-T-G-T-A-C-G-A-G-C-A-G-A-C-A-T-G-C-,transversion 颠换,碱基对的置换(substitution),移码突变 (frameshift mutation),Base-pair substitution,Silent
4、 mutations,Nonsense mutations,Missense mutations,沉默突变,无义突变,错义突变,Phenotypic effects 表型效应,Base pair deletion or insertion,nonsense mutations,无义突变,missense mutations,错义突变,Phenotypic effects 表型效应,missense mutations,错义突变,An example of mutation 突变的一个例子,Three important sources of mutations 三种重要的突变源,Inaccur
5、acy in DNA replication and meiotic recombination The chemical damages to DNA The class of insertions generated by transposons（转座子）,Spontaneous mutation 自发突变,Alterations in DNA sequence caused by spontaneous errors in DNA replication or meiotic recombination 指DNA复制或减数分裂的重组中自发产生的错误引起的生物DNA序列的改变,The oc
6、currence rate of spontaneous mutations is very low, depending on the different genes. 自发突变发生率很低，而且不同基因的发生率也不同 Virus and bacterial genes 病毒和细菌基因 1/1010 1/108 Human genes 人类基因 1/10 6 1/105 Mouse genes 小鼠基因 1/10 5 1/104,Replication fidelity 复制忠实性,1. DNA polymerase （DNA 聚合酶）,2. Repair system 修复系统,Proper
7、 base pairing， proofreading ，mismatch repair，RNA priming,The perpetuation of the genetic materials from generation to generation depends on maintaining rates of mutation at low level.遗传物质代代相传依赖于突变概率维持在低水平上。 However, if the genetic materials were perpetuated with perfect fidelity, the genetic variati
8、on needed to drive evolution would be lacking, and new species would not have arisen. 然而，传给子代的遗传物质如果绝对精确的话，就失去了驱动进化的基因变异，新的物种也不会形成。,Three important sources of mutations 三种重要的突变源,Inaccuracy in DNA replication and meiotic recombination The chemical damages to DNASpontaneous hydrolysis and deaminationM
9、utagenesisPhysical mutagenesisChemical mutagenesis The class of insertions generated by transposons,DNA damage spontaneously from hydrolysis and deamination,Depurination sites 脱嘌吟位点,A, G,C or T,Break of the glycosylic bond causes the lost of bases , leaving the sites free of bases, to which any nucl
10、eotides could be inserted during DNA replication,Mutagenesis 诱变-1,Physical mutagenesis 物理诱变Physical mutagens 物理诱变剂Ionizing radiation break of DNA double strands, which is difficult to be restored.X- and -rayNonionizing radiation molecular vibration or the promotion of electrons to higher energy leve
11、ls, leading to the formation of new chemical bonds.UV,Mutagenesis caused by radiation is nonspecific and occurs at random.,Mutagenesis 诱变-2,Chemical mutagenesis 化学诱变 Chemical mutagens 化学诱变剂 Base analogs: derivatives of the normal bases with altered base pairing properties. Direct mutagenesisNitrous
12、acids deamination of cytosine, adenine Alkylating and arylating agents lesionIndirect mutagenesisIntercalators insertion and deletion mutation,Base analogs 碱基类似物,5-溴尿嘧啶 (5-BU) 5-溴去氧尿核苷 (5-BudR) 2-氨基嘌吟(2-AP),5-BU can pair to both A and G, causing error in base pairs, which will result in transition.,
13、Alkylating agents 烷化剂,Guanine is ethylated into 6-ethylguanine，which can base-pair with thymine,EMS甲基磺酸乙酯，硫酸二乙酯（DES）等，带有一个或多个活泼的烷基，能碱基形成烷基化碱基，改变氢键的结合能力，从而改变碱基的配对。,Acridine dyes insert to DNA,B-DNA before intercalation,B-DNA after intercalation,Intercalating agents,2-氨基吖啶、吖啶橙、ICR-170等能嵌入DNA双链中的碱基之间，引
14、起单一核酸的缺失或插入，造成突变。,UV and high energy radiation 紫外线和高能量放射物,Three important sources of mutations 三种重要的突变源,Inaccuracy in DNA replication and meiotic recombination The chemical damages to DNA The class of insertions generated by transposons（转座子）,DNA repair DNA 修复,Photoreactivation 光复活Alkyltransferase re
15、pair 烷基转移酶修复Excision repair 切除修复Mismatch repair 错配修复SOS repair SOS 修复Homologous recombination 同源重组,How to find? How to repair?,Photoreactivation 光复活作用,Cleavage of the cyclobutane ring (环丁烷环) of pyrimidine dimers by DNA photolyses restores the original DNA structure.,Error-free direct reversal 无错直接逆转
16、,Alkyltransferase repair 烷基转移酶修复,The alkyl group is removed from alkylated base by alkyltransferase and transfered to the enzyme itself.,Error-free direct reversal无错直接逆转,Base excision repair 碱基切除修复,AP site,Base excision repair,Nucleotide excision repair 核苷酸切除修复,Excinuclease （UvrABC）,Nick,Nick,DNA po
17、lymerase I,DNA ligase,Excision endonuclease，a nuclease (enzyme) which excises a fragment of nucleotides during DNA repair,12-13nt,Mismatch repair 错配修复,GATC CTAG,A T,GATC CTAG,A C,GATC CTAG,A T,GATC CTAG,A T,Mismatch repair,a, unmethylated GATC is 5 of mutation,b, unmethylated GATC is 3 of mutation,S
18、OS repair 紧急修复,DNA recombination,Homologous recombination 1 同源重组1,recBCD, site”（chi site）5-GCTGGTGG-3, 同源分子联会 DNA sequence are identical or nearly identical for a region of at least 100 bp or so., DNA断裂,Homologous recombination 2 同源重组 2,3,Strand exchange is called “strand invasion” , accomplished by
19、 recA protein, strand-exchange protein,链侵入,Homologous recombination 3 同源重组3,连接,Homologous recombination 4,Heteroduplex region forms (B - b and B -b),5,分支移位,异源双链螺旋,Homologous recombination 5 同源重组5,Holliday structure,ruvC resolves structure,Homologous recombination 6 同源重组6,Both strands are different,O
20、ne region different on one strand,crossover product,noncrossover product,Major biological roles of homologous recombination,To repair double-stranded breaks (DSBs) in DNA DNA断裂的修复 To promote genetic exchange between the chromosome of a cell and DNA that enters the cell via phage-mediated transductio
21、n or cell cell conjugation促进细菌染色体与外来的DNA的遗传交换（通过噬菌体介导的传导及细胞与细胞接合的方式进入细胞）,In bacteria,Major biological roles of homologous recombination,To repair DNA break and collapsed replication forks DNA断裂的修复和复制叉的重启 To promote genetic exchange and chromosome maintenance 遗传交换和染色体维持 To be essential to the process
22、 of chromosome pairing during meiosis 减数分裂中的染色体配对所必须。,In eukaryotic cells,Site-specific recombination 位点特异性重组,Phage,E. coli,Site-specific recombination 位点特异性重组,Int （intergrase 整合酶）- encoded by phage,IHF （integration host factor 整合宿主因子） - encoded by E. coli,Xis (excisionase 切除酶）-encoded by phage,C-C
23、B-B,int binding sites,Recombination sites involved in intergration and excision,Transposition 转座作用,A specific form of genetic recombination that moves certain genetic elements from one DNA site to another. The mobile genetic elements are called transposable elements or transposons. 是一种特殊的遗传重组，它是将特定的
24、遗传因子从DNA的一个地方移位到另一个地方。不依赖于DNA分子之间序列同源性的非位点特异性重组。,Transposition 转座作用,Insertion Sequences,Host DNA,Target site,Inverted terminal repeat,Inverted terminal repeat,Target repeat,Target repeat,Transposase （转座酶）,TACGT,ATGCA,123456789 123456789,987654321 987654321,DNA polymerase I DNA ligase,（DNA聚合酶I、DNA连接酶）,transposase,Questions,Define the following terms：mutation, DNA damage, Point mutation, Transition, Transversion, Frameshift , mutagenesis, homologous combination, transposition Describe the main kinds of DNA repairs and their major mechanisms.,
