DNA

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DNA Repair

• ??????? DNA ??????????
• ?????????? original state

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Mechanisms ??? DNA Repair

• I. True repair ????? defective DNA ???????
• 1. Direct reversal mechanism ????
• Directly undoing DNA damage
• 2. Excision repair
• 3. Mismatch repair
• II. Non-repair method ??????? defective DNA
• 4. Recombination repair
• 5. Error-prone repair system

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1. Direct Reversal Mechanism

• ????????????????????????? ????Directly undoing
  DNA damage
• 1.1 Light-dependent repair /
• Photoreactivation
• 1.2 O6- methylguanine methyltransferase

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1.1 UV damage repair / Light repair

• Pyrimidine dimer (TT) ????????????? UV
  ???????????????????????? ???????? (light gt 300
  nm)
• ??? photoreactivating enzyme ???? DNA photolyase
• ??? activate enzyme ?????? TT dimer ???
• ???? bacteria, microbial eukaryote ??????

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1.2 Direct reversion ??? Methylation / Ethlyation

• ??????? DNA ??? ??? methylation ???? ethylation
• O6- methylguanine methyltransferase ??? methyl
  ???? ethyl group ??? Guanine ?? DNA
• Enzyme ????? sulfur atom ??? cystein ??????????
  methyl ???? ethyl ???? enzyme ???????????? ????
  suicide enzyme ????????????????????
• ??????????? E. coli ??? human

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O6- methylguanine methyltransferase
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2. Excision Repair

• ???????????????? damaged DNA ?????????? fresh DNA
  segment ?????????
• Excision repair ???? repair ????????????????
  ?????????????? mechanisms
• 2.1 Base excision repair
• 2.2 Nucleotide excision repair

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2.1 Base excision repair

• ??? damaged base ?????????? DNA glycosylase
• ??? AP site
• 1) DNA glycosylase ??? damaged base ??????
• glycosidic bond ??? sugar ???????? base AP
  site
• (Apurinic ???? Apyrimidinic sites)
• 2) AP endonuclease ??? Phosphodiesterase ???
• sugar-phosphate group ???
• 3) DNA polymerase ??? base ??????????
• 4) DNA ligase ?????? 2 nucleotides ?????????

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• Abnormal base
• Glycosylase ??? base --gt AP site
• AP endonuclease ??? sugar-PO4
• DNA polymerase refills new base
• DNA ligase seals nick

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2.2 Nucleotide excision repair

• ??? abnormal bases ??????? ???? TT dimers ????
  base ????? side group ????????
• ???? E. coli ??? human
• 1) Excision nuclease (Exonuclease) ???
  oligonucleotide (12 mers) ????? damaged bases ???
  
• 2) DNA polymerase I fills gap
• 3) DNA ligase seals nick

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• Excision nuclease ????????????? 3 genes ???
• UvrA,
• UvrB ???
• UvrC

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3. Mismatch Repair

• Mismatch ??????????? replication ?????
• (1) ??? base ???
• (2) failure ??? proofreading system
• ??????? repair ???? replication ???? T G
  ?????????
• Parental strand ?? base ????
• Progeny strand ?? mismatched base
• ???????????????? methylation ??? base ?? ???
  pogeny ???????????????????????????? T ???? G ???

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• ??? A ?? GATC sequence ????? methylated ??? 250
  base pairs ???? replication
• ???????????? methylation ????? pogeny --gt
  mismatch repair system ??????????????????????
  methyl ??? mismatched base ????????? progeny ????
  ??????? methylate
• Prokaryote ??? Eukaryote ?? mismatch repair
  system ????????

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• Mismatch repair ?? E. coli ?? proteins ??? 4
  genes
• mutH, mutL, mutS ??? mutU ( uvrD)
• MutH protein ?? GATC-specific endonuclease
  activity ?????? GATC ??? ?????? methylated ????
  5 ???? 3 ?? 100 nt --gt ??? gap
• DNA pol fills gap ???
• DNA ligase seals nick

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4. Recombination Repair(Postreplicative Repair)

• Repair mechanism ?????????????? ???????????? DNA
  replication ???? ??? repair ??? ??? system
  ?????????????
• ????? DNA pol III replicate ????? TT dimer
  ????? DNA replication ???????????? ????
  replication ??????? dimer ???? primer ???? ???
  duplex

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• ???? Replication --gt 2 ??? sister - double
  helices
• Damaged double helix Sister double helix
  (normal) parent strand ?? TT (1) normal parental
  strand (3) progeny strand ?? gap (2) normal
  progeny strand (4)

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• Recombination repair ???????? ??? progeny ????
  gap
• Gapped strand (2) ??? damaged duplex ?? cross ???
  
• parent strand (3) ??? normal double duplex
• ?????? nucleotides ??? (3) ???????? gap ??? (2)
• ??? T T (1) ?????? ????????? repair ????????
  nucleotide
• excision repair system ???? ????? repair
• ??? gapped (2) ???????
• Parent strand (3) ??? sister duplex ?? gap --gt
  filled ???? DNA pol I DNA ligase --gt sister
  duplex (3 4) ???????

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• Recombination ??????? RecA protein ??? recA gene
• RecA protein ????????? exchange single strand
  ??????? homologous double helices ?? homologous
  recombination ????????

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5. Error-prone (SOS) Repair

• ????????????????????? ? ??? mutagents ???? UV
  light ?????????????????????????? ??? SOS response
• SOS response ?????????? DNA repair ???????,
  recombination, ?????????? proteins ????????
  replication
• Error-prone repair
• ???? ???????????? SOS response
• ???? replication ??? DNA pol ?????? base ???????
  damaged segment ??? template strand
• ????????????? 2 proteins ??? umuC umuD (UV
  mutable) genes

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• DNA damage ??????? induce pathway
• (1) mutagen ???? UV damages DNA --gt activate
  recA gt RecA ???? protease ?????????? targets
• (2) RecA ???? LexA ???? repressor ???? genes
• --gt LexA ??????? genes ??? repressed ???????????
  ???? umuDC operon
• (3) umuDC operon active --gt transcription --gt
• translation ??? UmuD UmuC
• (4) UmuD UmuC ????? DNA pol ??????? ? damaged
• site replication ????? ?????? base ???
  ??????? GC ??????? TT ??? replication
  ??????????????

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• Error-prone repair ???? ???????????? replication
  ??? damaged sites ?????? base ???
• UV activates RecA
• RacA cleaves LexA
• umuDC operon trnscribes
• UmuC UmuD ?????? DNA replicate ??? bases ??????
  T T dimer

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DNA Damage, Repair Cell Function