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  4. If E.coli is allowed to grow for 40 minutes in a medium containing N15, then the number of N14 N14 containing DNA would be:

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

KCETKCET 2017Molecular Basis of Inheritance

Solution:

The experiment performed by Messelson and Stahl proves that the replication pattern of the DNA is semi-conservative. The DNA helix is composed of two strands.
During replication, the strands separate and the new strands are formed on the original template strands.

One of the strand remains the older and one new strand is synthesized and the process is known as semi-conservative. Since, the cells are grown in medium containing $N ^{15}$ so there will be no strand with $N ^{14} / N ^{14}$.