Question

Meselson and Stahl used ____________ as the medium for density gradient centrifugation.

• A. Caesium chloride
• B. Calcium chloride
• C. Ammonium chloride
• D. Copper sulphate

Answer 1

The correct option is A Caesium chloride

Meselson and Stahl utilised the technique of density gradient centrifugation in their experiments. They grew E.coli in a medium containing ${}^{15}$$N{H}_{4}Cl$. Consequently, all the DNA molecules had ${}^{15}$N, the heavier isotope of nitrogen, incorporated into them.
After that, the cells were transferred into a medium containing normal $N{H}_{4}Cl$. The cells were isolated at different time intervals and their double stranded DNA was extracted. These strands were separated using CsCl density gradient centrifugation.

Density gradient centrifugation is a technique that helps in the separation of substances based on density.
When the isolated DNA was ultracentrifuged in the CsCl density gradient, the molecules with different densities started separating and forming discrete bands in the centrifugation tube. This helped to conclude the type of nitrogen incorporated in it and hence predict the nature of DNA replication.