Chapter 12_post

Genetics: A Conceptual Approach

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Unformatted text preview: Chapter 12 DNA Replication Dr. Ed Otto George Mason University 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 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 ______ ______________ 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 Watson and Crick (1953) 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. 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 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) 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 Meselson-Stahl Experiment (1958) 15 N only 15 N & 14 N 14 N only H-H H-L L-L Lighter Heavier Density Gradient 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 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 Semiconservative Replication Parent strands...
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This note was uploaded on 05/06/2010 for the course BIO 311 taught by Professor Otto during the Spring '10 term at George Mason.

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Chapter 12_post - Chapter 12 DNA Replication Dr. Ed Otto...

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