Here's why:

* DNA contains phosphorus: DNA is a nucleic acid, and its backbone is made of a sugar-phosphate chain. Phosphorus is a key component of this structure.

* Proteins contain sulfur: Proteins are made up of amino acids, and some amino acids contain sulfur.

* Selective labeling: By using ³²P, they could specifically label the DNA of the bacteriophage (virus).

* Tracing the material: After the bacteriophages infected bacteria, Hershey and Chase found that the ³²P-labeled DNA was transferred into the bacteria, while the protein coat labeled with sulfur-35 (³⁵S) remained outside. This showed that DNA was the genetic material being passed on.

Let me know if you have any other questions about the Hershey-Chase experiment!