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Lecture 13

Lecture 13 - replication repair - BIOL308.pdf

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BIOL 308
Bruce Wolff

Lecture 13Repair of Replication errorsProofreading mechanism of replisome 13 to 5 exonuclease activity of polymeraseRemoves wrongly incorporated basesImproves fidelity but mismatches still occurMismatch Repair 5Mismatch repair removes errors that escape proofreading corrects errors during replicationIncreases accuracy of DNA repair by 1001000xMust be rapidbefore the next replication roundMust findandcorrectlyreplace mismatchDistortion of DNA backbone enables detection of mismatch Mismatch repair systemmutgenes of Ecoli 5MutSscans DNA for distortionBinds to mismatchMutL recruited by MutSmismatchDNA complexMutS translocates along DNA until a GATC sequenceis reachedRequires ATPBinding of MutS at the 2 locations creates loop in DNAMutSL activates MutHMutHendonucleaserecognizes GATC binds MutSLMutHnicks unmethylated strand of DNALewin 2006 Fig 2014Strand is then progressively degraded from GATC to mismatchIn 53 direction or in 3 to 5 direction using different exonucleasesA helicase UvrD assistsNew DNA strand is synthesizedDNA polymerase IISealed by DNA ligaseMismatch removed replaced with correctly basepaired nucleotideHow does repair system known which mismatched nucleotide should be replacedThe Dam methylase4GATC sequencesoccur once every 256 bp 4 except inDam methylasewill methylate the adenine of all GATC sitesHowever for a few min after the replication fork passes newly synthesized DNA is still hemimethylatedOnce activated by MutSL MutH selectively nicks unmethylated strandNew strand where the error is is selectively targeted NOTES FROM LECTURES Page 1
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