NEET Botany Molecular Basis of Inheritance Questions Solved


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

Messelson and Stahl's experiment: 

(i) They grew E.coli in a medium containing N15H4Cl as the only nitorgen source for many generations. The result was that N15 was incorporated into newly synthesised DNA.

(ii) The heavy DNA molecule could be distinguished  from the normal DNA by centrifugation in a cesium chloride density gradient.

   (Separation of DNA by centrifugation)

   Messelson's and Stahl's experiment 

(iii) They transferred the cells into a medium with normal N14H4Cl and took samples at various definite time intervals as the cells multiplied, and extracted the DNA that remained as doubled-stranded helicals.  

(iv) Thus, the DNA that was extracted from the culture one generation after the transfer from N15 to N14  medium (i.e., E.coli divides in 20 minutes) had hybrid or intermediate density.

(v) DNA extracted from the culture after another generation (that is after 40 minutes, II generation) was composed of equal amounts of the hybrid DNA and of light DNA.                                             (3)

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