Question

What do you mean by semi-conservative replica? Describe the experiment, performed by Matthew Meselson and Franklin Stahl to prove that DNA replicates semi-conservatively. Draw the diagram of semi-conservative DNA replicate model.

Answer 1

Semiconservative means both the strand of DNA separate to each other and act as template new strand is formed on this two template strand in DNA replication. 

Meselson and stahl performed following experiment in 1958 - 

(i) They grew E. coli in a medium containing ¹⁵NH₄ Cl(¹⁵N is the heavy isotope of nitrogen) as the only nitrogen source for many generations. The result was that ¹⁵N 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 (Please note that 15N is not a radioactive isotope, and it can be separated from ¹⁴N only based on densities). 

(ii) 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 (Figure).

	(Separation of DNA by centrifugation)

(iii) Thus, the DNA that was extracted from the culture one generation after the transfer from ¹⁵N to ¹⁴N medium [that is after 20minutes; E. coli divides in 20minutes] had a hybrid or intermediate density. DNA extracted from the culture after another generation [that is after 40minutes, II generation] was composed of equal amounts of this hybrid DNA and of ‘light’ DNA.