Final answer:
The radioisotope sulfur-35 (³⁵S) effectively labels phage protein without labeling DNA, as shown in the Hershey and Chase experiments, which confirmed DNA as the genetic material rather than protein.
Step-by-step explanation:
A radioisotope of an element that would effectively label phage protein but not DNA is sulfur-35 (³⁵S). This choice is based on the molecular composition of proteins and nucleic acids. As demonstrated by Hershey and Chase in their historic experiments, sulfur is present in the amino acids methionine and cysteine, which are building blocks of proteins, but is not found in nucleic acids such as DNA.
Their experiment involved creating batches of phages with radiolabeled components: one with radioactive phosphorus (³²P), which labels DNA, and another with ³⁵S, which labels protein. After allowing these phages to infect E. coli cells, they observed that only ³²P entered the bacterial cells. The ³⁵S remained in the phage remnants, proving that protein, labeled by ³⁵S, was not the genetic material transferred during infection. This confirmed that DNA, and not protein, was the genetic material responsible for directing the infection process of the bacterium by the phage.