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Q.
E. coli with completely $N^{15}$ DNA was allowed to replicate in $N^{14}$ medium for two generations. Percentage of bacteria with heavy DNA will be
NTA AbhyasNTA Abhyas 2022
Solution:
Meselson and Stahl found that the DNA of the first generation of bacteria was hybrid or intermediate $\left( N _{14} N _{14}\right)$. The second generation of bacteria, after $40$ minutes, contained two types of DNA, $50\%$ light ( $\left.N _{14} N _{14}\right)$ and $50 \%$ intermediate $\left( N _{14} N _{15}\right)$. The third generation of bacteria after $60$ minutes contained two types of DNA, $25\%$ intermediate ( $\left.N _{15} N _{14}\right)$ and $75 \%$ light $\left(N_{14} N_{14}\right)$ in a $1: 3$ ratio. The fourth-generation after $80$ minutes contained $12.5 \% N _{15} N _{14}$ and $87.5 \% N _{14} N _{14}$ DNA in a $1:7$ ratio.