Question

Escherichia coli fully labelled with ${}^{15}N$ allowed to grow in ${}^{14}N$ medium. The two strands of $DNA$ molecule of the first generation bacteria have

• A. different density and do not resemble parent $DNA$
• B. different density but resemble parent $DNA$
• C. same density and resemble parent $DNA$
• D. same density but do not resemble parent $DNA$

Answer 1

The correct option is B different density but resemble parent $DNA$

When E. coli fully labelled with $N_{15}{N}_{15}$ is allowed to grow in $N_{14}{N}_{14}$ medium, then after fist genertion of replication one of the two strands would have $N_{15}$ and the other strand would have $N_{14}$. The two strands of DNA molecule of the first generation bacteria have different density and resemble parent DNA. The resulting molecule would have a density which is intermediate between $N_{15}{N}_{15}DNA$ and $N_{14}{N}_{14}DNA$. These two molecules of $DNA$ will be similar but not same in density. In 1958, Meselson and Stahl proved the semi-conservative nature of DNA replication in E. Coli through this experiment.