Hershey and Chase intended to see if it's the viral DNA or the viral protein coat that enters bacteria upon infection with bacteriophage (virus infecting bacteria). By determining this they could tell which component of virus actually carries the genetic information and results in formation of more viruses with in the bacterium that they have infected.
With this intention in mind they took two sets of bacteriophages (called T2 virus). One set had its DNA labeled with radioactive phosphorus and the other set had its protein coat labeled with radioactive sulfur. These two sets were allowed to infect E Coli bacteria separately.
After infecting with the bacteriophages, the two batches of E Coli were mixed in a blender to shove off any viral remnants attached to the bacterial surface.
Each mixture was subjected to centrifuge to separate the heavier bacteria in the form of a pellet from lighter viral remnants.
Radioactivity was checked in both the batches. In sulphur labeled group the radioactivity was detected in the supernatant-the lighter liquid. This meant that the sulphur coated protein coat of the bacteriophages didn't enter the bacterial cells. On the other hand, the phosphorus coated group showed radioactivity in the pellet - the bacterial residue.
This experiment showed that its the viral DNA that enters the bacterial cell and multiplies with in to form more viruses.
This solution explains how Hershey-Chase experiment made use of radioactive isotopes to demonstrate that DNA, not protein, is the genetic material.