Meselson and Stahl experiment
Meselson and Stahl grew bacteria (Escherichia coli) in a medium with heavy nitrogen (N15). N15 is a heavier isotope of nitrogen than usual, N14. Although it has the same electrons (7) and protons (7) as N14, its weight is greater, since it has one more neutron (8). Thus, this isotope was incorporated into the DNA chains that were synthesized, making them heavier.
The bacteria were then transferred to a lighter 14N medium where they continued to grow.
Since the DNA molecules synthesized with N15 weigh more than those of N14, they can be separated by centrifugation, since the less heavy DNA molecules are higher and the heavier DNA molecules are lower.
In the first generation of bacteria, a single DNA band with intermediate density was obtained. In the second generation, two bands were obtained, one with a light density and the other with an intermediate or hybrid density. In the third generation, two bands were obtained, one light (with an abundance of 75%) and another intermediate (with the remaining 25%).
The intermediate or hybrid band represents a DNA molecule that contains a heavy chain (original, with N15) and a light chain (recently synthesized, with N14). The light chains represent a DNA molecule in which the two chains have been synthesized (they did not exist even when the cells were put in the presence of N15).
The fact that there are more and more light molecules and the number of intermediate molecules is being maintained shows that DNA replication is semi-conservative. If I were conservative, a heavy band would always appear and the rest. If it were dispersive, only hybrid bands of intermediate density would appear in all generations.