Meselsonâ€“Stahl experiment

The biologist Matthew Meselson and Franklin Stahl developed in the 1958 published and named after them Meselson -Stahl experiment, which can be demonstrated that the replication of DNA semi- conservative (half - preserving ), the genotype of the daughter cells after cell division so each is half of the genetic information of the parent cell and is synthesized de novo in half.

In addition to the semi- conservative before the hypotheses of the conservative and the disperse replication were discussed:

When conservative replication, the parent DNA is fully retained and copies of their two single strands come together to form a new double strand.
In the semi- conservative replication, the parent DNA remains half in each daughter molecule. The other half is newly added.
The disperse replication (also dispersive replication ) extends in principle similar, again half of the parent DNA is in each daughter DNA obtained, the other half is replaced by new nucleotides. However, the replacement mechanism is completely different. The nucleotides of the parent DNA alternate namely from the newly added nucleotides.

For their experiment, the researchers first cultured bacteria on a nutrient medium containing only one nitrogen isotope with mass 15 u contained. This was then integrated by the bacteria in their DNA. Subsequently, bacteria of this strain were applied to a nutrient medium which u contained nitrogen with a mass of 14. After 20 minutes, then bacteria of the first next generation ( F1 generation ) were collected and subjected her genetic material of a density gradient. It was found that the Sedimentationsebene the bacterial DNA exactly between the reference planes of DNA that u nitrogen contained only 14 or 15, was. This allowed the conservative theory be excluded, since otherwise two Sedimentationsebenen at the level of the reference planes would have had to make.

In order to decide which of the other two theories is correct, the process was repeated with individuals of the F2 generation. It was found that the genetic material of bacteria sedimented half in the plane of the F1 generation and half in the 14 u - reference plane. As the whole sediment between the F1 plane and the u - 14 level would have in a just disperse replication must be deposited, but the result corresponds exactly to the predictions of the semi- conservative theory, this is proved by the experiment.