First experimental proof for semi-conservative DNA replication was shown in

Options

A. Streptococcus pneumoniae
B. Escherichia coli
C. Neurospora crassa
D. Rattus rattus

Explanation

To solve the question regarding the first experimental proof for semi-conservative DNA replication, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Semi-Conservative DNA Replication**:
- Semi-conservative replication means that when DNA replicates, each new double helix consists of one old (parental) strand and one newly synthesized strand.

2. **Identify Key Experiments**:
- The first experimental proof of semi-conservative DNA replication was provided by the Meselson-Stahl experiment.

3. **Know the Organisms Involved**:
- The Meselson-Stahl experiment was conducted using Escherichia coli (E. coli), a type of bacteria.

4. **Evaluate the Options**:
- The question provides four options:
- A) Streptococcus pneumoniae
- B) Escherichia coli
- C) Neurospora crassa
- D) Rattus rattus
- From our understanding, the correct answer is B) Escherichia coli.

5. **Conclude the Answer**:
- Therefore, the first experimental proof for semi-conservative DNA replication was shown in Escherichia coli.

### Final Answer:
**The first experimental proof for semi-conservative DNA replication was shown in Escherichia coli (E. coli).**

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