Question

In their experiment, Meselson and Stahl cultured E.coli in a medium containing

• A. ${}^{15}N$
• B. ${}^{35}S$
• C. ${}^{32}P$
• D. Both B and C

Answer 1

The correct option is A

${}^{15}N$

Meselson and Stahl cultured E.coli in a medium of ammonium chloride $\left({}^{15}N{H}_{4}Cl\right)$, containing heavy nitrogen ${}^{15}N$. They used heavy nitrogen so that the bacteria grown in such a medium would contain denser DNA than those grown in a medium containing normal nitrogen $\left({}^{14}N\right)$. This difference in densities would result in separation of the DNA strands containing heavy nitrogen from those containing light nitrogen, during centrifugation. This could help them figure out which nitrogen $\left({}^{14}Nor{}^{15}N\right)$ was actually getting incorporated into the DNA molecules.