Describe Meselson and Stahl's experiment that was carried in 1958 on E.Coli. Write the conclusion they arrived at after the experiment. (5 marks)
Messelon and Stahl's experiment :
(i) They grew E.coli in a medium containing 15NH4Cl as the only nitrogen source for many
generations. The result was that 15N 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 14NH4Cl and took samples at various
definite time intervals as the cells multiplied, and extracted the DNA that remained as
(iv) Thus, the DNA that was extracted from the culture one generation after the transfer from 15N
to 14N 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.