Q25 of 51 Page 1

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

The steps of the Meselson and Stahl’s experiment:

(i) They grew E. coli for many generations in 15NH4Cl to get the heavy isotope 15N as the nitrogen source.


(ii) They incorporated 15N into the DNA.


(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 of 14NH4Cl with 14N as the nitrogen source.


(v) Samples were taken from this medium of 14NH4Cl and the DNA was extracted.


Observation:


DNA extracted from the culture after one generation (20 minutes is the generation time of E.coli) showed intermediate hybrid density. The DNA extracted after two generations (40 minutes) showed light DNA and hybrid DNA.


Conclusion:


It can be concluded that the newly synthesised DNA contains one strand of its parent and one is new. Thus, replication was semi-conservative.



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