Question

State whether true or false:
Meselson and Stahl distinguished ${}^{15}N$ from ${}^{14}N$ by means of radioactivity of the former.

• A. True
• B. False

Answer 1

The correct option is B False

^{${}^{15}N$} is not a radioactive isotope. ${}^{15}N$ and ${}^{14}N$ were separated in Meselson and Stahl’s experiment by centrifugation in a cesium chloride $\left(CsCl\right)$ density gradient. The heavier DNA molecules were the ones into which ${}^{15}N$ had got incorporated, while the lighter DNA molecules were the ones which had ${}^{14}N$ in them. As a result of ultracentrifugation, the heavy DNA molecules would settle at the bottom while the lighter DNA molecules would remain on top.