Experiment demonstrating the semi-conservative mode of DNA replication

DNA replication is the process by which DNA molecules are copied in living organisms. The question of how DNA replicates was initially answered by an experiment carried out by Matthew Meselson and Franklin Stahl in 1958. This experiment provided evidence for the semi-conservative mode of DNA replication, which means that each newly synthesized DNA molecule consists of one old (parental) strand and one newly synthesized strand.

The Experimental Setup:
1. Meselson and Stahl grew E. coli bacteria in a medium containing heavy isotope-labeled nitrogen (15N). This heavy isotope was incorporated into the DNA of the bacteria as they replicated.
2. After several generations, the bacteria were transferred to a medium containing normal nitrogen (14N) and allowed to replicate DNA for a specific period of time.
3. Samples were taken at different time intervals and DNA was extracted from the bacterial cells.

The Experimental Procedure:
1. The extracted DNA samples were subjected to density gradient centrifugation using a cesium chloride (CsCl) density gradient.
2. The DNA samples were loaded onto the gradient and centrifuged at high speeds for several hours.
3. As a result of centrifugation, the DNA molecules separated into distinct bands based on their densities. The heavier DNA molecules settled lower in the gradient while the lighter ones stayed higher up.
4. The DNA bands were then carefully collected and analyzed using a technique called autoradiography.

The Results:
1. After one round of DNA replication in the presence of 14N, a single band of intermediate density was observed.
2. This intermediate band indicated that the DNA molecules were composed of a mix of heavy (15N-labeled) and light (14N) DNA, supporting the hypothesis of semi-conservative replication.
3. After the second round of replication, two bands were observed: one corresponding to the intermediate density and another at the lighter density.
4. This result indicated that each DNA molecule consisted of one parental heavy strand and one newly synthesized light strand, providing further evidence for semi-conservative replication.

The Conclusion:
The Meselson-Stahl experiment provided strong evidence for the semi-conservative mode of DNA replication. It demonstrated that during replication, the parental DNA strands separate and serve as templates for the synthesis of new complementary strands. Each resulting DNA molecule contains one original (parental) strand and one newly synthesized strand. This experiment laid the foundation for our understanding of DNA replication and its significance in inheritance and genetic diversity.