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

1. different density and do not resemble parent DNA
2. different density but resemble parent DNA
3. same density and resemble parent DNA
4. same density but do not resemble parent DNA

Correct answer: different density but resemble parent DNA

Solution

In semiconservative replication, the original 15N strand serves as a template for a new 14N strand. So the first-generation DNA molecules are hybrids with intermediate density, but they still carry one parental strand and therefore resemble the parent DNA in composition.

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