Use of ${ }^{15} \mathrm{~N}$ will be inappropriate because method of detection of ${ }^{35} \mathrm{P}$ and ${ }^{15} \mathrm{~N}$ is different $\left({ }^{32} \mathrm{P}\right.$ being a radioactive isotope while ${ }^{15} \mathrm{~N}$ is not radioactive but is the heavier isotope of Nitrogen). Even if ${ }^{15} \mathrm{~N}$ was radioactive then its presence would have been detected both inside the cell ${ }^{15} \mathrm{~N}$ incorporated as nitrogenous base in $DNA$) as well as in the supernatant because ${ }^{15} \mathrm{~N}$ would also get incorporated in amino group of amino acids in proteins). Hence the use of ${ }^{15} \mathrm{~N}$ would not give any conclusive results.