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

The following steps were followed in the experiment: [2]
(i) E. coli was grown in a medium containing ${}^{15}N{H}_{4}CI$ the heavy isotope ${}^{15}N$ as the sole nitrogen source which led to the incorporation of ${}^{15}N$ into the newly synthesised DNA which ultimately made the DNA heavy.
(ii) This heavy DNA was separated from the normal DNA by density gradient centrifugation using caesium chloride as the gradient.
(iii) The cells were then transferred into a medium with ${}^{14}N$ as the nitorgen source.
(iv) Samples were taken from this medium affer every 20 minutes and the DNA was extracted.

Observation: [1]
(i) Since E.coli divides every 20 minutes, the DNA extracted after 20 minutes in the experiment had a hybrid density.

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



Conclusion: [2]

(i) DNA extracted from the culture after another generation was composed of equal amounts of this hybrid DNA and of light DNA.

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