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Escherichia coli fully labelled with $15 \mathrm{~N}$ is allowed to grow in $14 \mathrm{~N}$ medium. The two strands of $DNA$ molecule of the first generation bacteria have
different density and do not resemble with their parent $DNA.$
different density but resemble with their parent $DNA.$
same density and resemble with their parent $DNA.$
same density but do not resemble with their parent $DNA.$
Solution
Messelson and Stahl $(1958)$ cultured (Escherichia coli) bacteria in a culture medium containing $15 \mathrm{~N}$. After these had been replicated for a few generations in the medium both the strands of their DNA contained $15 \mathrm{~N}$ as constituents of purines and pyrimidines. When these bacteria with $15 \mathrm{~N}$ were transferred in cultural medium containing $14 \mathrm{~N}$, it was found that $DNA$ separated from fresh generation of bacteria possesses one strand heavier than the other. The heavier strand represents the parental strand and lighter one is the new one synthesized from the culture indicating semi conservative mode of $DNA$ replication.
