Semiconservative replication of DNA was first demonstrated in

Options

A. Streptococcus pneumoniae
B. Salmonella typhimurius
C. Drosophila melanogaster
D. Escherichia coli

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the first demonstration of semiconservative replication of DNA.

2. **Identifying Key Scientists**: The experiment that demonstrated semiconservative replication was conducted by scientists Matthew Meselson and Franklin Stahl.

3. **Organism Used in the Experiment**: Meselson and Stahl performed their experiment using the bacterium *Escherichia coli* (E. coli).

4. **Experimental Design**: They cultured *E. coli* in a medium containing nitrogen-15 (N-15), which is a heavy isotope of nitrogen. This labeling allowed them to track the DNA.

5. **Labeling of DNA**: The DNA of the *E. coli* became heavy due to the incorporation of the N-15 isotope. After a generation, they shifted the bacteria to a medium containing nitrogen-14 (N-14), a lighter isotope.

6. **Observing Results**: After one round of replication in the N-14 medium, the DNA consisted of one heavy strand (N-15) and one light strand (N-14), demonstrating that each strand of the original DNA served as a template for a new strand.

7. **Conclusion**: The results showed that DNA replication is semiconservative, meaning that each new DNA molecule consists of one old (parent) strand and one new (daughter) strand.

8. **Final Answer**: Therefore, the semiconservative replication of DNA was first demonstrated in *E. coli*.

---