Question

Describe the experiment that helped demonstrate the semi-conservative mode of DNA replication.

Answer 1

Experiment

The experiment was performed by Meselson and Stahl. The following steps were followed in the experiment:

• E. coli was grown in a medium containing the heavy isotope ¹⁵N as the sole nitrogen source. This led to the incorporation of ¹⁵N into the newly synthesised DNA, which ultimately made the DNA heavy.
• This heavy DNA was seperated from the normal DNA by density gradient centrifugation using cesium chloride as the gradient.
• The cells were then transferred into a medium with ¹⁴N as the nitrogen source. Samples were taken from this medium and the DNA was extracted.

Observation

Since E. coli divides every 20 minutes, the DNA extracted after 20 minutes in the experiment had a hybrid density.

The DNA extracted after 40 minutes had equal amounts of hybrid and light densities.

Conclusion

This implies that the newly synthesised DNA obtained one of its strands from the parent. Thus, replication was semi-conservative.