During Messelson and Stahl experiment, heavy DNA was distinguished from normal DNA by centrifugation in

Meselson and Stahl subjected the DNA isolated from E.coli after certain period of incubation at the beginning in ${ }^{15}{\mathrm{NH}}_4\mathrm{Cl}A$ and later in ${ }^{14}{\mathrm{NH}}_4\mathrm{Cl}$ to CsCl gradient centrifugation to suspend the heavy DNA at the bottom, lighter DNA at the upper level and hybrid DNA at the median position of centrifuge