question

describe the experiment where mathew meselson and franklin stahl used heavy isotope of nitrogen.

 

 

 

 

 

 

matthew meselson and franklin stahl experiment:

in this experiment, theygrew e. coli in a medium containing 15nh4cl (15n is the heavy isotope of nitrogen) as the only nitrogen source for many generations. the result was that n was incorporated into newly synthesised dna (as well as other nitrogen-containing compounds). this heavy dna molecule could be distinguished from the normal dna by centrifugation in a cesium chloride (cscl) density gradient.

then they transferred the cells into a medium with normal 14nh4cl and took samples at various definite time intervals as the cells multiplied, and extracted the dna that remained as double-stranded helices. the various samples were separated independently on cscl gradients to measure the densities of dna.

thus, the dna that was extracted from the culture one generation after the transfer from 15n to 14n medium [that is after 20 minutes; e. coli divides in 20 minutes] had a hybrid or intermediate density. dna extracted from the culture after another generation [that is after 40 minutes, ii generation] was composed of equal amounts of this hybrid dna and of ‘light’ dna. they concluded that dna replicates semi conservatively.

thus dna is semi-conservative in nature was proved by them.