If an E.coli with heavy DNA N¹⁵ is put in a medium with NH₄ Cl[N¹⁴] then, what type of DNA will be obtained after two generations/replications?

N¹⁴ N¹⁵ and N¹⁴ N¹⁴ type

The correct answer is Option (2) -N¹⁴ N¹⁵ and N¹⁴ N¹⁴ type

It is now proven that DNA replicates semiconservatively. It was shown first in Escherichia coli and subsequently in higher organisms, such as plants and human cells. Matthew Meselson and Franklin Stahl performed the following experiment in 1958:

(i) 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 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 .

(iii) 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.