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(B) The generation time of $E. coli$ is $20$ minutes.After $60$ minutes, the number of generations $(n)$ = $60 / 20 = 3$.Initially, there is $1$ molec

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$25$

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(B) The generation time of $E. coli$ is $20$ minutes.After $60$ minutes, the number of generations $(n)$ = $60 / 20 = 3$.Initially, there is $1$ molecule of light $DNA$ $(^{14}N-^{14}N)$.After $1$ generation: $2$ molecules ($1$ hybrid, $1$ light is not possible, both become hybrid $^{14}N-^{15}N$).Actually, in semi-conservative replication, the original strands remain as templates.After $n$ generations, the total number of $DNA$ molecules = $2^n = 2^3 = 8$.The number of $DNA$ molecules containing original light strands = $2$ (since only the two original strands from the parent cell will remain as light templates).Percentage of light $DNA$ = $(	ext{Number of light } DNA / 	ext{Total } DNA) 	imes 100 = (2 / 8) 	imes 100 = 25 \%$.

An $E. coli$ cell with light $DNA$ is incubated in a culture medium containing a heavy isotope of nitrogen $(^{15}N)$. After $60$ minutes, find out the percentage of light $DNA$. (in $\%$)

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