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DNA Replication Models

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After Watson and Crick proposed the double helix structure for DNA, three models for DNA replication were proposed: conservative, semiconservative, and dispersive. In the conservative model, the first replication produces one daughter molecule containing both parental strands and another containing all new strands. In the semiconservative model, each parental strand is a template for a new strand. Each daughter molecule has a parental strand and a new strand. In the dispersive model, each parental strand is broken into double-stranded segments that act as templates for new molecules. The segments then reassemble into complete double helices, each with parental and new segments interspersed.

1. Suppose you were a scientist evaluating these three models when they were first proposed. What would have been your hypothesis regarding which model was correct? Explain
2. Which two models have similar results after replication cycle 1?
3 Why were culture samples taken after each replication cycle?
4. Which model(s), if any, is ruled out by the experimental results of replication cycle 1? Explain.
5. Which model is proved correct by the experimental results of replication cycle 2? Explain.

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1. Based on the lack of knowledge about DNA replication at the time when these 3 models were proposed, any three of the DNA replication models could be correct at the time. However, through the work of two scientist named Matthew Meselson and Franklin Stahl, they proved that DNA replication is semiconservative. They used two isotopes of nitrogen, 15N and 14N to label the E. coli DNA and they used a technique known as cesium chloride (CsCl) equilibrium density gradient centrifugation as their sedimentation method. They used nitrogen to label DNA because it was a central component of DNA. They knew that when a cell divides and replicate its DNA, it incorporates new N atoms into the DNA of either one or both of its two daughter cells. The CsCl density equilibrium gradient centrifugation will separate different density DNA molecules. In other words, the DNA labeled with the heavy nitrogen will resolved different from the lighter nitrogen. By centrifugation, they found the E.coli had DNA containing the heavy nitrogen 15N appeared as a single band when grown in 15N medium for several generation. They then transfer the ...

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