Semiconservative replication of DNA was first shown in

Options

A. Drosophila melanogaster
B. Escherichia coli
C. Diplococcus pneumoniae
D. Neurospora cassa

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks which organism was first used to demonstrate the semi-conservative replication of DNA. The options provided are Drosophila melanogaster, Escherichia coli, Diplococcus pneumoniae, and Neurospora crassa.

2. **Defining Semi-Conservative Replication**: Semi-conservative replication is a method of DNA replication where each of the two strands of the DNA molecule serves as a template for the production of a complementary strand. This results in two DNA molecules, each containing one original strand and one newly synthesized strand.

3. **Identifying Key Experiments**: The key experiment that demonstrated semi-conservative replication was conducted by scientists Matthew Meselson and Franklin Stahl. They used Escherichia coli (E. coli) for their experiments.

4. **Experimental Procedure**: Meselson and Stahl cultured E. coli in a medium containing a heavy isotope of nitrogen (15N) to label the DNA. After several generations, they shifted the bacteria to a medium containing a lighter isotope (14N) and allowed the bacteria to replicate.

5. **Results of the Experiment**: After one round of replication, the DNA was found to be of intermediate density, indicating that each DNA molecule consisted of one heavy strand (from the 15N) and one light strand (from the 14N). This result was inconsistent with the conservative model of replication, which would predict the presence of both heavy and light DNA.

6. **Conclusion**: The results from the Meselson-Stahl experiment provided strong evidence for the semi-conservative model of DNA replication. Therefore, the correct answer to the question is **Escherichia coli**.

**Final Answer**: The semi-conservative replication of DNA was first shown in **Escherichia coli**.

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