A DNA molecule of Escherichia coli is heavy (full labelled with N15) and is allowed to duplicate in a medium containg N14. In first generation, the two daughter DNA molecules:-

Options

A. have same density but do not resemble their parent DNA
B. have different densities but resemble their parent DNA
C. have different densities and also do not resemble the parent DNA
D. same in density and resemble their parent DNA

Answer

To solve the question regarding the DNA duplication of Escherichia coli in a medium containing N14 after being initially labeled with N15, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Initial Condition**:
- The original DNA molecule of Escherichia coli is fully labeled with N15, which is a heavy isotope of nitrogen. This means that the DNA is denser than normal DNA, which contains N14.

2. **Setting Up the Experiment**:
- The DNA is allowed to duplicate in a medium that contains only N14. This means that during the replication process, the new nitrogenous bases that are added to the growing DNA strands will be made from N14.

3. **DNA Replication Process**:
- During the first round of replication, each strand of the original N15-labeled DNA will serve as a template. As the DNA strands separate, new complementary strands will be synthesized using the available N14 from the medium.

4. **Formation of Daughter DNA Molecules**:
- After replication, each of the two new daughter DNA molecules will consist of one original strand (which is N15) and one newly synthesized strand (which is N14). This results in two hybrid DNA molecules:
- One strand from the parent DNA (N15)
- One newly synthesized strand (N14)

5. **Analyzing the Daughter DNA Molecules**:
- Both daughter DNA molecules are hybrids, meaning they contain one strand of N15 and one strand of N14.
- Although both daughter DNA molecules have the same density due to the presence of one heavy and one light strand, they do not resemble the parent DNA molecule, which was entirely made of N15.

6. **Conclusion**:
- Therefore, in the first generation, both daughter DNA molecules have the same density but do not resemble their parent DNA.

### Final Answer:
Both daughter DNAs have the same density, but they do not resemble their parent DNA.

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