Question

Describe Meselson and Stahl’s experiment that was carried in 1958 on E.Coli. Write the conclusion they arrived at after the experiment.

Answer 1

Experiment
The experiment was performed by Meselson and Stahl. The following steps were
followed in the experiment:
(i) E. coli was grown in a medium containing the heavy isotope 15N as the sole nitrogen source.
(ii) This led to the incorporation of 15N into the newly synthesised DNA, which ultimately made the DNA heavy.
(iii) This heavy DNA was separated from the normal DNA by density gradient centrifugation using caesium chloride as the gradient.
(iv) The cells were then transferred into a medium with 14N as the nitrogen source.
(v) Samples were taken from this medium and the DNA was extracted.

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