How is DNA conservative

During replication, the genome is reproduced. At the beginning of the 1950s, however, it was still unclear what principle the replication was based on. Three possible models have been controversially discussed in biology.

Conservative replication: The original DNA is retained in its form. Two new single strands are synthesized, which together form a double strand.

The semi-conservative replication: A new daughter strand is synthesized for each parent strand. Half of the DNA is retained and the other half is formed anew.

The disperse replication: The daughter molecules consist of an alternating mixture of old and newly synthesized strands.

Matthew Meselson (* 1930) and Franklin Stahl (* 1929), two American geneticists, experimented with Escherichia coli bacteria to uncover the riddle of replication.

To do this, they placed the bacteria on a nutrient medium that contained the nitrogen isotope 15N. This resulted in the isotope being incorporated into the DNA of the bacteria. After a while, they transferred the bacteria to a nutrient medium with the nitrogen isotope 14N. The only difference between the two isotopes is their mass, and Meselson and Stahl took advantage of this: They extracted the DNA of the next generation and centrifuged the genetic material. The result (one band) was exactly between the density gradients of 14N and 15N, which excluded conservative replication. Otherwise the centrifuged genetic material would have had to split up into density gradients 14N and 15N because the parental generation (P1) of the bacteria had built in 15N and the first branch generation (F1) could only have built in 14N.

This meant that only semiconservative or disperse replication came into consideration. The experiment was continued by a further generation of branches (F2). But this time you got two bands of different severity consisting of a light 14N band and a heavier 14N + 15N band. Disperse replication was also excluded, because in this case only one band should have formed (due to the constant incorporation of 14N and 15N).