DNA replication is said to be semiconservative'. Why? Describe the experimental proof of Meselson and Stahl to _x000D_show DNA replication is semiconservative.

Explanation

In both the daughter DNA molecules one strand is parental and another is newly synthesized. Hence, DNA replication is called Semi Conservative.
It was first shown by Matthew Meselson and Franklin Stahl (1958) in Escherichia coli and subsequently in higher organism. It is popularly known as Meselson and Stahl Experiment.
(a) They grew E.coli in NH4Cl medium for many generations. (TMN is heavy isotope of nitrogen)
(b) The result was that .N was incorporated into newly synthesized DNA and other nitrogen containing compound as well.
(c) This heavy DNA molecule could be distinguished from normal DNA by centrifugation in a caesium chloride (CsCl) density gradient.
(d) Then they transferred the E.coli into a medium with normal `""^(15)NH_4Cl` and let them grow.(E.coli divides in 20 minutes)
(e) They took samples at definite time intervals as the cells multiplied, and extracted the DNA that remained as double-stranded helices.
(f) Various samples were separated independently on CsCl gradients to measure the densities of DNA.
(g) The DNA that was extracted from the culture one generation after the transfer from `""^15N` to `""^14N` medium had a hybrid or intermediate density.
(h) DNA extracted from the culture after another generation(after 40 min). was composed of equal amount of this hybrid DNA and light DNA.one generation after the transfer from ..N to "N medium had a hybrid or intermediate density. (h) DNA extracted from the culture after another generation (after 40 min.) was composed of equal amount of this hybrid DNA and of .light .DNA.