E. coli bacteria grew in ${}^{15}N{H}_{4}Cl$ medium for several generation are allowed to grow in ${}^{14}N{H}_{4}Cl$ medium. After 2 generations, bacteria are isolated from medium and DNA of bacteria centrifuged in CsCl. The result of density gradient consists of DNA that is

Meselson-Stahl showed the semi-conservative mechanism of DNA replication. They cultured E. coli cells initially in a medium containing ammonium salts prepared with heavy nitrogen ${}^{15}$N15 to label, using all cellular DNA. Then cells were transferred to a medium containing the normal light isotope ${}^{14}$N14. DNA was analyzed by equilibrium density-gradient centrifugation in the periodically collected sample to separate heavy-heavy (H-H), light-light (L-L), and heavy-light (H-L) duplexes into distinct bands. The sample was placed on a preformed gradient of cesium chloride. Centrifugation separates the macromolecules in different bands of density gradient which can be collected as a pure fraction. After the first generation, the density gradient contained only one band having H-L nitrogen. After the second generation, the density gradient contained 2 bands- one will have hybrid DNA (H-L) and the other will have light DNA (L-L).

Thus, the correct answer is option D.