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Chapter 16 DNA REPLICATION
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REVIEW: HISTORY & STRUCTURE
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GRIFFITH’S EXPERIMENT
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HERSHEY & CHASE’S EXPERIMENT
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AVERY, M C CARTY, & MACLOED’S EXPERIMENT
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WILKINS & FRANKLIN’S EXPERIMENT
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CHARGAFF’S EXPERIMENT
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WATSON & CRICK
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Making DNA from existing DNA Semi- conservative At the end of DNA replication, each daughter molecule has one old strand (from the parent DNA) and one new strand (synthesized during replication) DNA REPLICATION
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Step 1: Helicases unwind DNA at origin of replication by breaking hydrogen bonds between nitrogen bases Replication bubble forms as two parental strands separate Replication fork forms at end of each replication bubble DNA REPLICATION
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Step 2: Single-strand binding proteins hold the unpaired DNA strands apart while new DNA strands are being synthesized Topoisomerase protein binds to parental DNA to relieve strain untwisting puts on replication fork DNA REPLICATION
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Step 3: Primase creates a short RNA primer that binds to the parent DNA to signal DNA polymerase III where to begin adding nucleotides RNA primer will later be replaced with DNA nucleotides DNA REPLICATION
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Step 4: DNA Polymerase III adds nucleotides to exposed bases in 5’-3’ direction at the RNA primer Leading strand Produced continuously in 5’-3’ direction Elongation moves towards replication fork Lagging strand Produced in pieces Okazaki fragments Elongation moves in opposite direction of replication fork (5’-3’)
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Step 5: Lagging strand is completed as DNA ligase seals Okazaki fragments
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Initial error rate in replication is 1 in 100,000 nucleotides DNA polymerases proofread and correct errors Error rate in completed replication is 1 in 10 billion bases PROOFREADING & REPAIR
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For that 1 in 10 billion errors that escapes DNA polymerase or are due to environmental mutations Many enzymes involved Cut out damaged section (nuclease) Replace with new nucleotides (DNA polymerase) Seal in place (DNA ligase) MISMATCH REPAIR
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As cells divide, chromosomes erode after multiple DNA replications Telomeres End caps of non-coding DNA that protect the coding genes on the chromosomes Think of the plastic pieces on the ends of your shoelaces These sequences shorten after each round of replication so the actual genes do not Part of aging process? TELOMERES
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Enzyme that lengthens the ends of telomeres in germ cells (give rise to gametes) Prevents loss of critical genes in zygote Cancer cells have mechanisms that activate telomerase to allow unlimited division without loss of DNA TELOMERASE
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Bioflix animation Bioflix animation DNA replication cartoon DNA replication cartoon ANIMATIONS
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EXTENSIVE REPLICATION http://www.wiley.com/college/pratt/0471393878/student/an imations/dna_replication/index.html http://www.wiley.com/college/pratt/0471393878/student/an imations/dna_replication/index.html
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CHROMOSOME ANYONE? http://www.biostudio.com/d_%20Replication%20of%20a%20 Chromosome.htm http://www.biostudio.com/d_%20Replication%20of%20a%20 Chromosome.htm
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