 Meselson and Stahl grew bacteria in a growth medium that contained a heavy nitrogen isotope, 15N. These bacteria were then transferred to a medium with normal light nitrogen, 14N. After one generation, the bacteria were harvested and their DNA was extracted and loaded onto a caesium chloride solution in the ultracentrifuge chamber (Fig. 2). Caesium atoms are so heavy that they will form a concentration gradient when subjected to high centrifugal forces. In such a gradient, DNA floats at a band in the gradient where the molecular density corresponds to that of the caesium chloride solution. Thus, the analytic centrifuge is an extremely sensitive measuring device for molecular density. In fact, it is sensitive enough to detect the difference between DNA that contains heavy nitrogen and DNA that contains light nitrogen. It can also distinguish these two DNA species from a hybrid made of one strand containing heavy nitrogen and one strand containing light nitrogen. Indeed, after one round of replication, the bacterial DNA in Meselson’s and Stahl’s experiment was of intermediate density. After another round, the DNA had the same density as DNA in its natural state.