Step 1: Understanding the Concept:
After the discovery of the double helix structure of DNA, there were three proposed models for how DNA replicates: Conservative, Semi-conservative, and Dispersive.
In the semi-conservative model, the two strands of the parent DNA separate, and each serves as a template for the synthesis of a new complementary strand.
As a result, each daughter DNA molecule contains one original (parental) strand and one newly synthesized strand.
While Watson and Crick proposed this model based on the structure of DNA, it required experimental evidence to be accepted by the scientific community.
Step 2: Detailed Explanation:
Matthew Meselson and Franklin Stahl performed a definitive experiment in 1958 using {Escherichia coli} to prove the semi-conservative nature of replication.
They used two isotopes of nitrogen: $^{15}N$ (a heavy, non-radioactive isotope) and $^{14}N$ (the common light isotope).
The experiment involved the following steps:
1. {E. coli} cells were grown in a medium containing $^{15}NH_{4}Cl$ for many generations until all their DNA was fully "heavy" ($^{15}N$).
2. These cells were then transferred to a medium containing normal $^{14}NH_{4}Cl$ (light nitrogen).
3. DNA was extracted from the cells at specific intervals corresponding to their doubling time (approximately 20 minutes).
4. The extracted DNA was analyzed using density gradient centrifugation in Cesium Chloride ($CsCl$).
- After the first generation (20 mins), the DNA formed a single band at an intermediate density, containing one strand of $^{15}N$ and one strand of $^{14}N$. This ruled out conservative replication.
- After the second generation (40 mins), two equal bands were observed: one intermediate (hybrid) and one light ($^{14}N/^{14}N$). This confirmed semi-conservative replication.
- In subsequent generations, the light band grew larger while the hybrid band remained constant in total amount but decreased in percentage.
Watson and Crick (A) only proposed the model; Hershey and Chase (C) proved DNA is genetic material; and Griffith (D) discovered transformation.
Step 3: Final Answer:
The use of heavy and light nitrogen isotopes to track the distribution of parental DNA among daughter molecules was the hallmark of the Meselson and Stahl experiment.
Final Answer: (B) Meselson and Stahl