Meselson–Stahl experiment

DNAreplicationModes

Meselson-stahl experiment diagram en chiral.svg

The Meselson–Stahl experiment was an experiment by Matthew Meselson and Franklin Stahl in 1958 which supported the hypothesis that DNA replication was semiconservative. This experiment is considered one of the most beautiful experiments in biology.

Background

The Watson and Crick model of DNA suggested that the two strands of the DNA double helix could serve as templates for the synthesis of new strands. However, it was not clear whether DNA replication was conservative, semiconservative, or dispersive.

Experimental Design

Meselson and Stahl grew Escherichia coli (E. coli) in a medium containing a heavy isotope of nitrogen, nitrogen-15 (15N). As the bacteria replicated their DNA, the nitrogen-15 was incorporated into the DNA, making it denser. They then shifted the bacteria to a medium containing the lighter isotope nitrogen-14 (14N) and allowed the bacteria to continue replicating.

Methodology

The DNA was extracted from the bacteria at various time intervals and subjected to density gradient centrifugation in a solution of cesium chloride. This technique allowed the separation of DNA based on its density.

Results

After one round of replication in the 14N medium, the DNA formed a single band at an intermediate density, indicating that each DNA molecule consisted of one strand of 15N DNA and one strand of 14N DNA. After two rounds of replication, two bands were observed: one at the intermediate density and one at the density corresponding to 14N DNA. These results were consistent with the semiconservative model of DNA replication, where each daughter DNA molecule consists of one old strand and one newly synthesized strand.

Conclusion

The Meselson–Stahl experiment provided strong evidence for the semiconservative mechanism of DNA replication. This experiment is a cornerstone in the field of molecular biology and has been fundamental in our understanding of genetic inheritance.

Legacy

The Meselson–Stahl experiment is often cited as a classic example of elegant experimental design and execution. It has been widely taught in biology and genetics courses and remains a pivotal study in the history of science.

See also

• DNA replication
• Watson and Crick model
• Density gradient centrifugation
• Escherichia coli