Assume That You Grew a Culture of E

question 13

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 sampleA).You then wash the remaining
E.coli cells,grow them for one generation in ¹⁴N,and extract the DNA from a portion of the culture (call this sampleB).You let the culture grow for one more generation in ¹⁴N and extract the DNA (call this sampleC)Each sample of DNA (A,B,and 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.)

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