The experiment that definitively demonstrated DNA as the molecule of heredity was conducted by Alfred Hershey and Martha Chase in 1952. This experiment is known as the Hershey-Chase experiment.

Here's a summary of their experiment:

* The Experiment: They used bacteriophages, viruses that infect bacteria. Bacteriophages are composed of a protein coat and a DNA core. They labeled the protein coat with radioactive sulfur (³⁵S) and the DNA core with radioactive phosphorus (³²P). They then infected bacteria with these labeled bacteriophages.

* The Results: They found that the ³²P (DNA) entered the bacteria, while the ³⁵S (protein) remained outside. The infected bacteria then produced new bacteriophages that contained the ³²P label.

* The Conclusion: This experiment showed that DNA, not protein, was the genetic material that was passed on from one generation to the next.

The Hershey-Chase experiment provided compelling evidence that DNA is the molecule of heredity, confirming earlier research by other scientists like Avery, MacLeod, and McCarty.