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Semiconservative DNA Replication: Meselson and Stahl Experiment

• Watson and Crick proposed a semiconservative model of DNA replication in which each parental strand separates and serves as a template for a complementary strand.
• The conservative model of replication predicts that the two original strands rewind after replication, while the dispersive model predicts interspersed sections of both old and new DNA in daughter strands.
• Meselson and Stahl performed an experiment using Escherichia coli cultured in a heavy growth medium with uniformly labeled 15N bases, followed by transfer to a light growth medium containing 14N.
• DNA samples were extracted at various generations and analyzed through cesium chloride equilibrium density gradient centrifugation.
• At generation 0, all bacterial DNA was uniformly labeled with 15N and migrated as a single band in the gradient.
• At generation 1, the semiconservative model predicted hybrid DNA molecules with one parental 15N strand and one new 14N strand, which was observed.
• The conservative model predicted two DNA molecules, one with two 15N strands and one with two 14N strands, which was ruled out.
• The dispersive model was not ruled out until generation 2, where the data was consistent with the semiconservative model and argued against the dispersive model.
• At generation 2, two bands containing equal quantities of DNA were observed, one with the predicted hybrid density and the other with the predicted density for DNA with two 14N strands.
• Generations 3 and 4 also supported the semiconservative model as the only one fully consistent with the data.
• Meselson and Stahl's experiment provided evidence for the semiconservative model of DNA replication and helped establish its validity.
• The experiment was performed just five years after the Watson-Crick model was proposed and demonstrated the importance of experimental evidence in validating scientific models.