Assume That You Grew a Culture of E

Essay

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/cm³ (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/cm³ (call this sample
B). You let the culture grow for one more generation in ¹⁴N, and extract the DNA (call this sample
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.)

C). Each sample of DNA (A, B, and