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Genetics: A Conceptual Approach

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Chapter 12 DNA Replication Dr. Ed Otto George Mason University
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Key objectives Understand requirements for replication and basic steps in the process Examine enzymes and proteins involved in DNA replication in bacteria Discuss differences in the replication process in eukaryotes
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DNA Replication Prior to cell division, DNA must be duplicated so that each offspring cell receives a copy of the genetic material Process of duplication is called ______ ______________
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DNA Replication Two key characteristics of the replication process 1. Replication must be ____________ - Given the huge amount of genetic material and the enormous number of cell divisions required to produce a multicellular organism, even a low rate of error during copying would be catastrophic 2. Replication must be _______ - The single chromosome in E. coli is 4.6 million base pairs - Bacterial cells divide every 20 minutes - Replication must proceed at more than 1000 base pairs per second
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Watson and Crick (1953)
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Watson and Crick (1953) “It has not escaped our notice that the specific pairing that we have postulated immediately suggests a possible copying mechanism for the genetic material.”
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Alternative Models for DNA Replication Watson-Crick structure suggested that new DNA chains were copied from old However, the mechanism for copying was unclear Three alternative models were proposed for how replication could occur: 1. ____________ replication 1. ___________ replication 1. ________________ replication
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Alternative Models of Replication • Entire original double helix serves as a template for synthesis of a whole new double helix • Original double helix is fully conserved (i.e. left intact) Two strands of original double helix break down into fragments that serve as templates for synthesis of new fragments • Fragments reassemble, with resulting double helices interspersed with old and new DNA • Each strand of the original double helix serves as a template for synthesis of a new strand • Resulting double helices are semi -conserved (i.e. one original and one new strand)
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Meselson-Stahl Experiment (1958) E. coli cells Grow in 15 N (heavy) medium Grow in 14 N (light) medium for one cell division for two cell divisions Isolate DNA and dissolve in CsCl Centrifugation
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Meselson-Stahl Experiment (1958) 15 N only 15 N & 14 N 14 N only H-H H-L L-L Lighter Heavier Density Gradient
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Predicted Outcomes Parent strands synthesized in 15 N 1 st doubling: New synthesis in 14 N 2 nd doubling: New synthesis in 14 N One band: H-H One band: H-H One band: H-H Two bands: H-H, L-L One band: H-L One band: H-L Two bands: H-H, L-L Two bands: H-L, L-L One band: H-L
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Experimental Results Parent strands synthesized in 15 N 1 st doubling: New synthesis in 14 N 2 nd doubling: New synthesis in 14 N One band: H-H One band: H-H One band: H-H Two bands: H-H, L-L One band: H-L One band: H-L Two bands: H-H, L-L Two bands: H-L, L-L One band: H-L Results
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Semiconservative Replication Parent strands synthesized in 15 N 1 st doubling: New synthesis in 14 N 2 nd doubling: New synthesis in 14 N One band: H-H One band: H-H One band: H-H Two bands: H-H, L-L One band: H-L
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