Assume that you grew a culture of
E. coli for many generations in medium containing
15N (from the ammonium ion), a heavy isotope of nitrogen. You extract DNA from a portion of the culture and determine its density to be 1.723 gm/cm
3 (call this sample A). You then wash the remaining
E. coli cells and grow them for one generation in
14N, extract the DNA from a portion of the culture, and determine its density to be 1.715 gm/cm
3 (call this sample B). You let the culture grow for one more generation in
14N and extract the DNA (call this sample C). Each sample of DNA (A, B, and C) is then heated to completely denature the double-stranded structures, cooled quickly (to keep the strands separate), and subjected to ultracentrifugation. Present the centrifugation profiles for heat-denatured DNA (samples A, B, and C) that you would expect. Use the graph below. (
Note: Although not the case, assume that single-stranded DNA has the same density as double-stranded DNA.)
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