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 as the only nitorgen source for many generations. The result was that 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 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 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)