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midterm pt2

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University of Toronto St. George
Kenneth Yip

Problem 1 Ends of DNA telomerase a Shortening of DNA in lagging strand lose info 2 Winding problem topoisomerase 3 Mistakes exonuclease 263288 Maintenance of DNA sequenceMutation rates are extremely lowLow mutation rates are necessary for life as we know it DNA replication mechanismsOrder of proteins required1 DNA helicaseUnwinds DNA after initiation protein replication bubble replication forkHelicase predominant53 o 6 identical subunits each bind ATP being hydrolyzed 2 Single stranded binding proteins binds to singlestranded DNAprevent singlestrands from reforming base pairs stabilize unwound singlestranded conformation of DNAHelixdestabilizing proteinsPlace holders 3 DNA primase 4 DNA ligase SemiconservativeOne strand in daughter molecule is conserved from parental moleculeother is newly synthesized bidirectional from 53 Not conservative both strands conserveddaughter consist 2 newly synthesized strandsBasepairing underlies DNA replicationrepairDNA replication forkasymmetricalHigh fidelity of DNA replication requires several proofreading mechanisms o RNA 1100000 DNA 110000000000 o 35 exonuclease activity part of DNA polymeraseImmediately after incorrect nucleotide is covalently added to growing chainIf incorrect nucleotide is added 3OH not properly positioned for continuing DNA synthesis polymerase will chip off previous nucleotide by adding correct one again o Stranddirected activitydouble stranded DNA takes away erroruse other strand as templateDetects distortion of DNA due to noncomplementary base pairsONLY DNA replication 53 direction allows efficient error correctionSpecial nucleotidepolymerizing enzyme synthesizes short RNA primer molecules on lagging strandSpecial proteins help open up DNA double helix in front of replication forkSliding ring holds moving DNA polymerase onto DNAThe proteins at replication fork cooperatedform replication machineStranddirected mismatch repair system removes replication errors escape from replication machineDNA topoisomerases prevent DNA tangling during replication o DNA not free to rotated entirely in front of replication forkpositively supercoiledunwinding DNA nrg costly o Topoisomerasechange supercoiled state of DNA duplex o Topoisomerase I tyrosine active sitebreaks Pdie bonds single stranded breaks rotation otherso Topoisomerase II double stranded breaks helices pass one another completely prevent tangling strandsDNA replication fundamentally similar in eucaryotesbacteria Initiationcompletion of DNA replication in chromosomes
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