A culture of E. coli growing in a medium containing a heavy isotope of nitrogen (15N) as the sole source of nitrogen is switched to a medium containing the normal, lighter isotope of nitrogen (14N) for 2 generations. What is the molar ratio of the hybrid DNA (15N-14N) to light DNA (14N-14N) at this point? Report your answer as a decimal to the nearest hundredths.
0.14 and 0.25 is not the answer
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All the DNA of E.Coli will contain N-15 initialy in N-15 medium. N-14 will be incorporated in the DNA When the Organism is shifted to N-14 medium. Hence the next generation will have one strand Containing N-15 And one strand containing N14. The ratio of N-14 to N-15 will be 1:1. In the next generation (Second from initial E. Coli) The new DNA strands will contain only N-14, But the parent will have one N-14 and one N-15 strand. Both N-14 and
N-15 will coil with new strands that contain N-14 strands only. So this generation will contain one N-15 strand for every three N-14 strands. Hence the ratio of N-15 to N-14 will be 1:3. (= 0.33)
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