Meselson and Stahl experimentally proved semi-conservative DNA replication.

Semi-conservative DNA replication:
Semi-conservative DNA replication is a process in which each new DNA molecule formed after replication contains one original parental strand and one newly synthesized daughter strand. This means that during replication, the parental DNA molecule separates into two strands, each serving as a template for the synthesis of a new complementary strand.

Meselson and Stahl experiment:

1. Aim:
The aim of Meselson and Stahl's experiment was to determine the mode of DNA replication, whether it is conservative, semi-conservative, or dispersive.

2. Experimental setup:
They used isotopes of nitrogen (15N and 14N) to label the DNA molecules. E. coli bacteria were grown in a medium containing 15N as the only nitrogen source, resulting in the incorporation of 15N into the DNA. After several generations, the bacteria were transferred to a medium containing 14N as the only nitrogen source.

3. Experimental procedure:
The bacteria were harvested at different time points after the transfer to 14N medium, and their DNA was extracted. The extracted DNA was then subjected to density gradient centrifugation using cesium chloride (CsCl) gradient.

4. Expected results:
If DNA replication was conservative, after one round of replication, there would be two distinct bands in the density gradient: a light band containing DNA with only 14N and a heavy band containing DNA with only 15N. If DNA replication was dispersive, there would be a single intermediate band containing a mixture of 14N and 15N DNA.

5. Actual results:
Meselson and Stahl observed that after one round of replication, there was a single intermediate band in the density gradient, indicating that the DNA molecules were composed of a mixture of 14N and 15N. This result ruled out both the conservative and dispersive models of DNA replication.

6. Conclusion:
Meselson and Stahl concluded that DNA replication is semi-conservative, as the observed intermediate band supported the presence of equal amounts of old and new DNA strands in each replicated molecule after one round of replication.

In summary, Meselson and Stahl's experiment provided experimental evidence for the semi-conservative mode of DNA replication, which is now widely accepted as the correct model for DNA replication.