(a) How did Meselson and Stahl reach the conclusion that DNA replication is semi-conservative while working with E.coli in their experiment ?

(b) Explain the contribution of Taylor and his colleagues in DNA replication in flowering plants.

 

 

Answers (1)

a) 

Matthew Meselson and Franklin Stahl performed the following experiment in 1958:

  •  They grew 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.

b) Taylor and his colleagues worked on Vicia faba /faba beans, using radioactive thymidine to prove that DNA In the chromosomes also replicates semi conservatively.

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