Question

Escherichia coli fully labelled with $N^{15}$ is allowed to grow in $N^{14}$ 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 not resemble parent DNA.

Answer 1

The correct option is B

Different density but resemble parent DNA.

When E. coli fully labelled with $N^{15}$ is allowed to grow in $N^{14}$ medium, then after first generation of replication one of the two strand would have $N^{15}$ and other strand would have $N^{14}$. The resulting molecule would have a density which is intermediate between $N^{15}$ DNA and $N^{14}$ DNA.