The Meselson-Stahl Experiment

To begin with, to be able to tell the old and new DNA apart, Meselson and Stahl grew several generations of Echerichia coli bacteria in two different mediums, one in the "heavy" nitrogen isotope ¹⁵N and the other in the "light" nitrogen isotope ¹⁴N. So, it turned out, that one bacterial culture contained the heavy form of nitrogen and the other contained the light form of nitrogen.

Then, taking samples of each and extracting DNA into solution, the scientists mixed the two DNA solutions together and added the mix to a CsCl (cesium chloride) solution of the same density. This mix was then processed at high speed in an ultracentrifuge. The result was a mixture separated by density—the "heavy" mix was denser than the "light" mix and so the "heavy" mix sank to the bottom and the "light" mix moved up. Thus it was possible to tell the two apart.

Next, the scientists grew several generations of E.coli in 'heavy' nitrogen medium, and extracted a DNA sample from these bacterial cells. They called this sample "generation zero" and prepared it for centrifugation. The result showed that the DNA in generation zero had a heavier density, that is was in the "heavy" form.

The scientists then transferred the E.coli bacterial cells to the "light" nitrogen medium and allowed it to grow in that. They took samples from this every 20 minutes and processed these samples too in an ultracentrifuge.

The first result, after one generation, showed that the DNA had an intermediate density rather than heavy or light. This ruled out conservative replication, as, in that case, the result wouldn't have shown intermediate density, but rather equal amounts of heavy and light DNA. On the other hand, intermediate density would be possible with both semiconservative and dispersive replication.

The second result, after two generations, showed that one part of the DNA had intermediate density and the other part had light density. This ruled out dispersive replication as in that case the DNA distribution would have been same between the strands and the resulting density would have been lower than the intermediate one.

The third result, after several more generations, showed that now a larger portion of the DNA had light density, synthesized from the first and second generations, and a smaller portion of DNA had intermediate density, sythesized from the heavy and light DNA. This proved the semiconservative hypothesis and was a major step in developmental biology research.