Assume that a culture of E. coli was grown for approximately 50 generations in 15N (provided in the medium in the ammonium ion), which is a heavy isotope of nitrogen (14N). You extract the DNA from the culture, and it has a density of 1.723 gm/cm3 (water = 1.00 gm/cm3). From the literature, you determine that DNA containing only the common form of nitrogen, 14N, has a density of 1.700 gm/cm3. Bacteria from the 15N culture were washed in buffer and transferred to 14N medium for one generation immediately after which the DNA was extracted and its density determined.

What would be the expected density of the extracted DNA?
What band(s) would be expected if subjected to ultracentrifugation through a CsCl gradient?
(b) How would your answer to (a) above differ if replication occurred in a conservative fashion?
(c) Assume that the extracted DNA was completely denatured (separated into two single strands by heating at 90oC for 15 minutes), what would be the density (or densities) of the DNA (s) in the heated extract?