The Meselson Steel Experiment

Attempt to prove the semiconservative replication

During replication, the genetic material is multiplied. At the beginning of the 1950's, however, it was still unclear what principle replication was. In biology, three possible models were discussed controversially.
Conservative replication: The original DNA is preserved in its form. Two new single strands are synthesized, which together form a double strand.
Semiconservative replication: A new daughter strand is synthesized into a parent strand. This leaves half of the DNA and the other half of it.
The disperse replication: The daughter molecules consist of an alternating mix of old and newly synthesized strings.
Matthew Meselson (b. 1930) and Franklin Stahl (b. 1929), two American geneticists, experimented with Escherichia coli bacteria to unravel the riddle of replication.
For this they put the bacteria on a nutrient medium that contained the nitrogen isotope 15N. This led to the incorporation of the isotope into the DNA of the bacteria. After a while they transferred bacteria to a nutrient medium with nitrogen isotope 14N. The two isotopes differ only in their mass and Meselson and Stahl took advantage of this: They extracted the DNA of the next generation and centrifuged the genome. The result (one band) was exactly between the density gradients of 14N and 15N, which precluded conservative replication. Otherwise, the centrifuged genome would have had to divide up to the density gradients 14N and 15N, because the parental generation (P1) of the bacteria had incorporated 15N and the first branch generation (F1) could have built only 14N.
Thus, only the semi-conservative or the disperse replication came into consideration. The experiment was continued by yet another branch generation (F2). But this time you got two different heavy bands consisting of a light 14N band and a heavier 14N + 15N band. Disperse replication was also excluded, as in this case only one band could be formed (due to the regular installation of 14N and 15N).