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

Options

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

Answer

To solve the question regarding the DNA of Escherichia coli (E. coli) that has been fully labeled with \(^{15}N\) and then allowed to grow in a medium containing \(^{14}N\), we need to analyze the process of DNA replication and the implications of using different nitrogen isotopes.

### Step-by-Step Solution:

1. **Understanding the Initial Condition**:
- E. coli is initially grown in a medium containing \(^{15}N\), which is a heavy isotope of nitrogen. This means that all the nitrogenous bases in the DNA are labeled with \(^{15}N\), making the DNA heavy.

2. **Transition to \(^{14}N\) Medium**:
- After the initial growth, the bacteria are transferred to a medium containing \(^{14}N\), which is a lighter isotope of nitrogen. During the next round of DNA replication, the bacteria will incorporate \(^{14}N\) into their new DNA strands.

3. **DNA Replication Process**:
- DNA replication in E. coli occurs via a semi-conservative mechanism. This means that each new DNA molecule consists of one old (parent) strand and one newly synthesized strand.
- The original DNA strands, which are both labeled with \(^{15}N\), will separate, and each will serve as a template for the synthesis of a new strand.

4. **Formation of New Strands**:
- The new strands will be synthesized using the \(^{14}N\) from the medium. Therefore, one strand will remain \(^{15}N\) (the parent strand), and the newly synthesized strand will be \(^{14}N\).

5. **Density Comparison**:
- The DNA that is composed of one \(^{15}N\) strand and one \(^{14}N\) strand will have a density that is different from the original \(^{15}N\) DNA. The \(^{15}N\) DNA is heavy, while the \(^{14}N\) DNA is light. The hybrid DNA (one strand of each type) will have an intermediate density.

6. **Final Conclusion**:
- In the first generation of bacteria, the two strands of DNA will have different densities (one heavy and one light) and will not resemble the original parent DNA in terms of isotopic composition. Thus, the correct answer is that the two strands of DNA have "different density and do not resemble parent DNA".

### Answer:
The two strands of DNA molecule of the first generation of bacteria have **different density and do not resemble parent DNA**.

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