Question

E. coli is grown exclusively in a medium containing \(^{15}NH_4Cl\) as the sole nitrogen source. Subsequently, the cells were shifted to a medium containing \(^{14}NH_4Cl\). The molar ratio of hybrid DNA (\(^{15}N-^{14}N\)) to light DNA (\(^{14}N-^{14}N\)) after four generations (rounded off to two decimal places) will be .......

Hint:

In a DNA labeling experiment, the ratio of hybrid to light DNA decreases with each generation as more new DNA is synthesized with light nitrogen (\(^{14}N\)).

Answer 1

In this experiment, E. coli is grown first in \(^{15}NH_4Cl\) (heavy nitrogen) and then shifted to \(^{14}NH_4Cl\) (light nitrogen). Initially, all the DNA is labeled with \(^{15}N\), and after the shift, new DNA is synthesized with \(^{14}N\).
After four generations, the following DNA types will be present:
- Generation 0: All DNA is heavy, \(^{15}N-^{15}N\).
- Generation 1: Half the DNA will be hybrid (\(^{15}N-^{14}N\)) and the other half will be light (\(^{14}N-^{14}N\)).
- Generation 2: Half of the hybrid DNA from Generation 1 will become \(^{14}N-^{14}N\), and the other half will remain hybrid (\(^{15}N-^{14}N\)).
- Generation 3: The ratio of hybrid DNA to light DNA will be 1:3.
- Generation 4: The ratio of hybrid DNA to light DNA will be 1:7.
Thus, after four generations, the molar ratio of hybrid DNA (\(^{15}N-^{14}N\)) to light DNA (\(^{14}N-^{14}N\)) is: \[ \frac{1}{7} \approx 0.14 \] Thus, the molar ratio of hybrid DNA to light DNA after four generations is \( \boxed{0.14} \).