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(A) Meselson and Stahl $(1958)$ cultured $Escherichia \,coli$ bacteria in a medium containing $15 \,N$ (heavy isotope). After several generations,both

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different density and do not resemble their parent $DNA$.

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(A) Meselson and Stahl $(1958)$ cultured $Escherichia \,coli$ bacteria in a medium containing $15 \,N$ (heavy isotope). After several generations,both strands of their $DNA$ contained $15 \,N$. When these bacteria were transferred to a medium containing $14 \,N$ (light isotope),the $DNA$ extracted from the first generation of bacteria showed an intermediate density. This is because each $DNA$ molecule consisted of one heavy parental strand $(15 \,N)$ and one newly synthesized light strand $(14 \,N)$. Therefore,the two strands have different densities and do not resemble the parent $DNA$ (which was entirely $15 \,N$). This experiment provided evidence for the semi-conservative mode of $DNA$ replication.

$15 \,N$ labeled $Escherichia \,coli$ is allowed to grow in a $14 \,N$ medium. The two strands of the $DNA$ molecule of the first generation bacteria have:

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