The Experimental Proof :-

The semi-conservative model, in which each strand of DNA serves as a template to make a new, complementary strand, seemed most likely based on DNA’s structure.

• They began by growing E. coli in medium, or nutrient broth, containing a “heavy” isotope of nitrogen N15.
• The bacteria took up the nitrogen and used it to synthesize new biological molecules, including DNA.
• The bacteria were switched to medium containing a “light” Nisotope and allowed to grow for several generations.
• Then measured the density of the DNA N and N using density gradient centrifugation. Lensity
• A dense solution of Cscl on centrifugation forms density gradient bands of solution of lower density at top that increase gradually towards bottom with highest density.

Results of the experiment

• When DNA from the first four generations of E. coli was analyzed, it produced the pattern of bands shown in the figure below:
• Thus the DNA that was extracted from culture after 1st generation i.e, just after 20 min had a hybrid or intermediate density DNA extracted from the culture after another generation i.e, 2nd generation or 40 min was composed of equal amounts of this hybrid.