If E. coli is allowed to grow for 40 minutes | vedclass

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(A) The correct answer is $A$ (zero).$E. coli$ replicates its $DNA$ every $20$ minutes.Initially,the $E. coli$ cells contain $^{14}N / ^{14}N$ $DNA$.W

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zero

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(A) The correct answer is $A$ (zero).$E. coli$ replicates its $DNA$ every $20$ minutes.Initially,the $E. coli$ cells contain $^{14}N / ^{14}N$ $DNA$.When these cells are transferred to a medium containing $^{15}N$,the new $DNA$ strands synthesized will incorporate $^{15}N$.After $20$ minutes (one generation),the $DNA$ molecules will be hybrid $(^{15}N / ^{14}N)$.After $40$ minutes (two generations),the $DNA$ molecules will be a mix of hybrid $(^{15}N / ^{14}N)$ and heavy $(^{15}N / ^{15}N)$.Since the medium only contains $^{15}N$,no new $^{14}N / ^{14}N$ $DNA$ can be formed,and the original $^{14}N$ strands will be paired with $^{15}N$ strands.Therefore,the number of $^{14}N / ^{14}N$ $DNA$ molecules will be zero.

If $E. coli$ is allowed to grow for $40$ minutes in a medium containing $^{15}N$,then the number of $^{14}N / ^{14}N$ containing $DNA$ molecules would be:

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