If DNA (or any other substance) is added, it concentrates and forms a band in the tube at the point where its density is the same as that of the CsCl. If several types of DNA with different densities are added, they form several bands. The bands are detectable under UV light at a wavelength of 260 nm, which nucleic acids absorb strongly.

Meselson and Stahl grew E. coli in a medium containing a heavy isotope of nitrogen (N15). After growing for several generations on the N15medium, the DNA of E. coli was denser. Then the culture was spun in a low-speed centrifuge, the heavy medium was discarded and the bacteria resuspended in a medium containing only N14.

To distinguish between Semiconservative and dispersive replication, E. coli culture was incubated for 40 min. instead of 20 minutes, so that two rounds of replication occurs. Again the cells were removed and their DNA was analysed in a density gradient. Two bands appeared, one representing the same hybrid molecules as before, the second corresponding to wholly N14 DNA. These results suggest that DNA replication is by Semiconservative mode

In contrast, the dispersive mode can be discarded because that method would still produce only hybrid molecules and in fact would continue to do so for a large number of cell generations.