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, 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) is then subjected to ultracentrifugation. Present the centrifugation profiles that you would expect under (a) semiconservative replication and (b) conservative replication. (Note: Assume that unlabeled [¹⁴N] DNA has a density of 1.700 gm/cm³.)
C). Each sample of DNA (A, B, and