Question

'DNA replicates semi conservatively'. Prove it with the help of Matthew Meselson and Franklin stall experiment. Draw the linear diagram of the experiment.

Answer 1

The following experiment suggests that DNA replication is semiconservative.

• Mattew Meselson and Frankin Stahi (1958) performed following using heavy nitrogen (${15}^N$) in E. coli.

1. They grew E. coli in a medium containing ${}^{15}N{H}_{4}Cl$ as the only nitrogen source for many generations. ${}^{15}N$ is the heavy isotope of nitrogen. It 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 caesium chloride (CsCl) density gradient. A dense solution of Cscl, on centrifugation, forms density gradient bands of a solution of lower density at the top that increase gradually towards the bottom with the highest density.
2. Then they transferred the cells into a medium with normal ${}^{14}N{H}_{4}Cl$ and took the sample at various definite time intervals as the cells multiplied, and extracted the DNA that remained as the double-standard helix. The various samples were separated independently on CsCl gradients to measure the densities of DNA.
3. Thus, the DNA was extracted from the culture after 1^st generation i.e. just after 20 minutes had a hybrid or intermediate density. DNA extracted from the culture after another generation i.e. 2^nd generation or 40 minutes was composed of equal amounts of this hybrid DNA ($N^{14}{N}^{15}$) and of light DNA ($N^{14}{N}^{14}$). Increase in the amount of light DNA and decrease in hybrid DNA amount can be possible due to the semiconservative mode of replication.