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  4. E.coli with completely N15 DNA was allowed to replicate in N14 medium for two generations. Percentage of bacteria with heavy DNA will be

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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 2020Molecular Basis of Inheritance

Solution:

Meselson and Stahl found that DNA of the first generation of bacteria was hybrid or intermediate (N15 N14). The second generation of bacteria, after 40 minutes, contained two types of DNA, 50% light (N14 N14) and 50% intermediate (N14 N15). The third generation of bacteria after 60 minutes contained two types of DNA, 25% intermediate (N15 N14) and 75% light (N14 N14) in 1:3 ratio. The fourth-generation after 80 minutes contained 12.5% N15 N14 and 87.5% N14N14 DNA in a 1:7 ratio.