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Question : Describe semiconservative replication of DNA step by step.

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Solution :It is proven that DNA replicates semiconservatively.
It was shown first in Escherichia coli and subsequently in higher organisms, such as plants and human cells.
Mathew Meselson and Franklin Stahl performed the following experiment in 1958 :
(i) They grew E. coli in a medium containing SNH,CI (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.

(ii) Then they transferred the cells into a medium with normal `""^(14)NH_(4)Cl` 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.
The results are shown in figure.
(iii) Thus, the DNA that was extracted from the culture one generation after the transfer from `""^(15)N` to `""^(14)N` 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.
Very similar experiments involving use of radioactive thymidine to detect distribution of newly synthesised DNA in the chromosomes was perfomed on Vicia faba (faba beans) by TAYLOR and Colleagues in 1958.
The experiments proved that the DNA in chromosome also replicate semiconservatively.


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