《分子生物学4 chapter9英语》由会员分享，可在线阅读，更多相关《分子生物学4 chapter9英语（42页珍藏版）》请在金锄头文库上搜索。
1、Chapter 9: Chapter 9: The mutability and repair of DNACausesRepairMutationRecombinationHomologous recombination (同源重组）同源重组）Site-specific recombination(位点专一重组位点专一重组) Transposition ( 转座）转座）The causes of mutationvMutagensvError in replicationchemical physicalEnsuring the accuracy Replication errors cau
2、se types Point mutation Insertions or deletionsn ntautomerism on base-pairingn nDNA polymorase IThe nature of mutationsPoint mutations: 1.Transitions (pyrimidine to pyrimidine, purine to purine)2.Transversions (pyrimidine to purine, purine to pyrimidine)Insertions or deletionsATG,GTG,GAG,CCG,GCC,GAG
3、,TAGATG,GTG,GAG,CTG,GCC,GAG,TAGATG,GTG,GAG,CCG,GCC,GAG,TAGATG,GTG,GAG,CG,GCC,GAG,TAGATG,GTG,GAG,CGG,CCG,AGT,AGPoint mutationInsertions or deletionsmissenselErrors in replicationReplication errors and their repairlRepair of Replication errors: Mismatch repair Increase the accuracy of DNA synthesis fo
4、r 2-3 orders of magnitudes.Prokaryote EukaryoteMismatch repair removes errors that escape proofreadingTwo challenges:1Rapidly find the mismatches/mispairs2 Accurately correct the mismatchProkaryote Mismatch repair systeml lMutSl lMutLl lMutHl lUrvDl lone of three exonucleases n nMutS MutS scansscans
5、 the DNA, the DNA, recognizingrecognizing the mismatch the mismatch from the distortion they cause in the DNA from the distortion they cause in the DNA backbone backbone n nMutS MutS embracesembraces the mismatch-containing DNA, the mismatch-containing DNA, inducing a pronounced kink in the DNA and
6、a inducing a pronounced kink in the DNA and a conformational change in MutS itselfconformational change in MutS itselfDNA is kinkedMutS is a dimer. One monomer interacts with the mismatch specifically, and the other nonspecifically.recruits recruits MutLMutL, , MutLMutL activates activates MutHMutH,
7、 ,enzyme causing an incision or nick on one enzyme causing an incision or nick on one strand near the site of the mismatch.strand near the site of the mismatch.Nicking is followed by the specific Nicking is followed by the specific helicasehelicase ( (UrvDUrvD) and one of three ) and one of three ex
8、onucleasesexonucleasesDifferent exonucleases are used to remove ssDNA between the nick created by MutH and the mismatch. recruits recruits MutLMutL, , MutLMutL activates activates MutHMutH, ,enzyme causing an incision or nick on one enzyme causing an incision or nick on one strand near the site of t
9、he mismatch.strand near the site of the mismatch.Nicking is followed by the specific Nicking is followed by the specific helicasehelicase ( (UrvDUrvD) and one of three ) and one of three exonucleasesexonucleasesExonuclease VII or IDNA polymerase IIIHow does the How does the E. coliE. coli mismatch r
10、epair mismatch repair system know which of the two system know which of the two mismatched nucleotide to replace? mismatched nucleotide to replace? DAM methylase methylate A residues on both strands of 5-GATC-3Replication generate hemimethylation DNA in E.coliMutH makes incision unmethylated daughte
11、r strandlErrors in replicationReplication errors and their repairlRepair of Replication errors: Mismatch repair Increase the accuracy of DNA synthesis for 2-3 orders of magnitudes.Prokaryote EukaryoteEukaryotic cellsEukaryotic cells also repair mismatches and do so using also repair mismatches and d
12、o so using homologshomologs to MutS (MSH) and to MutS (MSH) and MutLMutL (MLH). The underlying mechanisms are (MLH). The underlying mechanisms are not the same and not well understoodnot the same and not well understood. . DNA damage and their repairCausesRepairDirect reversal of DNA damageBase exci
13、sion repair Nucleotide excision repairRecombination (DSB) repairsTranslesion DNA synthesisdamage spontaneouslalkylation, oxidation and radiation base analogs intercalating agentsDNA undergoes damage spontaneously from hydrolysis and deaminationDeamination CUHydrolysis creates apurinic deoxyribose De
14、amination 5-mC T Explaining why DNA contains T instead of UAlkylation, OxidationNitrosamines (亚硝胺)Reactive oxygen species (O2-, H2O2, OH)Radiationbase analogspoint mutation intercalating agentsinsertion or deletionTwo consequence of DNA damagen nSome damages, such as thymine dimer, nick or breaks in
15、 the DNA backbone, create impediments to replication or transcriptionn nSome damages creates altered bases that has no effect on replication but cause mispairing, which in turn can be converted to mutation.DNA damage and their repairCausesRepairDirect reversal of DNA damageBase excision repair Nucle
16、otide excision repairRecombination (DSB) repairsTranslesion DNA synthesisdamage spontaneouslalkylation, oxidation and radiation base analogs intercalating agentsdamage spontaneouslalkylation, oxidation and radiation base analogs intercalating agentsDirect reversal of DNA damagePhotoreactivationMonom
17、erization of thymine dimers by DNA photolyases in the presence of visible light.Methyltransferase catalyzes the transfer of the methyl group on O6-methylguanine to cystein residue on the enzyme, thereby restoring the normal G in DNA.Methyl group removalBase Excision repairAP endonulease & exonulceas
18、e Cleaves the abasic sugarsDNA polymerase/ligase Works sequentially to complete the repair event.GlycosylaseRecognizes the damaged baseRemoves the damaged baseoxoG:A repair. A glycosylase recognizes the mispair and removes A. Fail-safe systemsNucleotide Excision repair1.Recognize distortions to the
19、shape of the DNA double helix2.Remove a short single-stranded segment that includes the lesion. 3.DNA polymerase/ligase fill in the gap.Nucleotide Excision-RepairUvrUvr A: A: detect the distortion of DNA detect the distortion of DNAUvrUvr B: B: melt DNA to create melt DNA to create ssDNAssDNA recrui
20、ts C recruits CUvrUvr C C create two incisions create two incisionsUvrUvr D D release the release the ssDNAssDNA Transcription-couple repair: nucleotideExcision repair system is capable of rescuing RNA polymerase that has been arrested by the presence of lesions in the DNA templateRecombination repa
21、irsDouble-strand break (DSB) repair pathwayFigure 10-4. Damage in the DNA template can lead to DSB formation during replicationFIGURE 10-3 DSB repair model for homologous recombination Translesion DNA synthesisOccurs when the above repairs are not efficient enough so that a replicating polymerase en
22、counters a lesion1.Translesion synthesis is catalyzed by a specialized class of DNA polymerases that synthesize DNA directly across the damage site. 2.Translesion polymerase is produced by cell in response to the DNA damage3.Translesion polymerases are expressed as part of the SOS response pathway. Translesion DNA synthesis in E. coli
