He further noted that if heat killed S strain was injected into a mouse, it did not cause pneumonia. Further experiments led Griffith to conclude that number 2 was correct. Their evidence was strong but not totally conclusive. The breakthrough in the quest to determine the hereditary material came from the work of Max Delbruck and Salvador Luria in the 1940s. Bacteriophage are a type of virus that attacks bacteria, the viruses that Delbruck and Luria worked with were those attacking Escherichia coli , a 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. Image from W.H. Freeman and Sinauer Associates, used by permission. 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.