Meselson and Stahl

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Matthew Meselson and Franklin Stahl are two biologists who prove that DNA replication was semiconservative.
At the time, many strong evidences from experiments using bacterial viruses had already convinced most scientists that DNA was the molecule of heredity; however they knew little about the DNA replication process. After the dimensionally accurate model building by Watson and Crick, it was clear that the process of replication and information distribution have to use the DNA from parent cell as template to achieve an orderly flow of genetic inheritance to daughter cells. However the model of Watson and Crick still cannot fill the missing information about how DNA might accomplish replication.
Before the Meselson-Stahl experiment,
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Meselson started the ultracentrifuge up with the sample taken just the medium had been switched. Photographic film showed a single sharp band, representing “pure heavy” DNA. He then began a run with sample drawn during the bacterial generation following the switch. The result of that run yielded two bands: an intense heavy band and a fainter band just to its lighter side. And, finally with the anticipated single band of heavy –light DNA.
Since conservative replication would result in equal amounts of DNA of the higher and lower densities but no intermediate density, therefore conservative replication was excluded.

However, this result was consistent with both semiconservative and dispersive replication. Semiconservative replication would result in double-stranded DNA with one strand of 15N DNA, and one of 14N DNA, while dispersive replication would result in double-stranded DNA with both strands having mixtures of 15N and 14N DNA, either of which would have appeared as DNA of an intermediate density.

Therefore they started to run the sample taken from second generations of growth, a band of representing “pure light” DNA was appearing in its proper place above the heavy-light band. The final result; the sample drawn at the end of the second generation, appeared two bands that visually to be equal intensity, in the position of
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