Here is a summary of the same experiments shown in the video. This information comes from your textbook "Reinforcements".
A series of experiments helped scientists recognize that DNA is the genetic material. One of the earliest was done by Frederick Griffith who was studying two forms of the bacterium that causes pneumonia. The "S" form was surrounded by a coating that made them look smooth. The "R" form did not have a coating, and the colonies looked rough. Griffith injected these bacteria into mice and found that only the "S" type killed the mice. When the "S" bacteria were killed, they did not cause the mice to die. However, when killed "S "bacteria were mixed with live "R" bacteria, the mice died and Griffith found live "S" bacteria in their blood. This led Griffith to conclude that there was a transforming principle that could change "R" bacteria into "S" bacteria.
Oswald Avery, another scientist, developed a way to purify this transforming principle. He then conducted a series of chemical tests to find out what it was. Many scientists thought that DNA was too simple of a molecule to be the genetic material and that proteins, being more complex, were a better candidate. However, Avery made three key discoveries about his samples of transforming principle that showed otherwise:
• DNA was present, not protein.
• The chemical composition matched that of DNA, not protein.
• The addition of enzymes that break down DNA made the transforming principle inactive. The addition of enzymes that break down proteins or RNA had no effect.
Alfred Hershey and Martha Chase carried out the final, conclusive experiments in 1952. Bacteriophages are viruses that infect bacteria and take over bacteria’s genetic machinery to make more viruses. They consist of a protein coat surrounding DNA. Hershey and Chase grew these viruses in cultures containing radioactively labeled sulfur, a component of proteins, or phosphorus, a component of DNA. Bacteria were then infected with viruses that either had radioactively labeled sulfur or phosphorous. Hershey and Chase next separated the viruses from the bacteria with a blender. The bacteria had radioactive phosphorus but no radioactive sulfur. Hershey and Chase concluded that the viruses’ DNA, but not the protein coat, had entered the bacteria.