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, and extract the dna from a portion of the culture (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 subjected to ultracentrifugation. label the centrifugation profiles for the below given diagram. (note: assume that unlabeled [14n] dna has a density of 1.700 gm/cm3.)