Griffith injected the different strains of bacteria into mice. He further noted that if heat killed S strain was injected into a mouse, it did not cause pneumonia. Bacteria recovered from the mouse had a capsule and killed other mice when injected into them. Their evidence was strong but not totally conclusive. Bacteriophage are a type of virus that attacks bacteria, the viruses that Delbruck and Luria worked with were those attacking Escherichia colia, bacterium found in human intestines.
Bacteriophages infect a cell by injecting DNA into the host cell.
After 25 minutes the host cell bursts, releasing hundreds of new bacteriophage. Phages have DNA and protein, making them ideal to resolve the nature of the hereditary material Alfred D. Hershey and Martha Chase conducted a series of experiments to determine whether the protein or DNA was the hereditary material. Conversely, protein lacks P but does have S, thus it could be tagged with radioactive Sulfur-35. Hershey and Chase found that the radioactive S remained outside the cell while the radioactive P was found inside the cell, indicating that DNA was the physical carrier of heredity. DNA had been proven as the genetic material by the Hershey-Chase experiments, but how DNA served as genes was not yet certain.