  Meselson and Stahl then produced a sample of E coli bacteria containing only \(\ce{^{15}N}\) by growing it in a medium containing only ammonium chloride with \(\ce{^{15}N}\) \((\ce{^{15}NH4Cl})\) for fourteen generations. They then abruptly changed the medium to \(\ce{^{14}N}\) by adding a tenfold excess of \(\ce{^{14}NH_4Cl}\). Samples were taken just before the addition of \(\ce{^{14}N}\) and at intervals afterward for several generations. The cell membranes were broken to release the DNA into the solution and the samples were centrifuged and ultraviolet absorption photographs taken. In addition, the photographs were scanned with a recording densitometer. The results are shown in Figure D, showing both the photographs and the densitometer traces. The figure shows that one starts only with heavy (fully-labeled) DNA. As time proceeds one sees more and more half-labeled DNA, until at one generation time only half-labeled DNA is present. “Subsequently only half labeled DNA and completely unlabeled DNA are found. When two generation times have elapsed after the addition of \(\ce{^{14}N}\) half-labeled and unlabeled DNA are present in equal amounts” (p. 676). (This is exactly what the semiconservative replication mechanism predicts). By four generations the sample consists almost entirely of unlabeled DNA. A test of the conclusion that the DNA in the intermediate density band was half labeled was provided by examination of a sample containing equal amounts of generations 0 and 1.9. If the semiconservative mechanism is correct then Generation 1.9 should have approximately equal amounts of unlabeled and half-labeled DNA, whereas Generation 0 contains only fully-labeled DNA. As one can see, there are three clear density bands and Meselson and Stahl found that the intermediate band was centered at \((50 \pm 2)\) percent of the difference between the \(\ce{^{14}N}\) and \(\ce{^{15}N}\) bands, shown in the bottom photograph (Generations 0 and 4.1). This is precisely what one would expect if that DNA were half labeled.