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