Griffith used two related strains of bacteria, known as R and S
R bacteria were nonvirulent, meaning that they did not cause sickness when injected into a mouse whereas mice injected with live S bacteria developed pneumonia and died
Griffith tried injecting mice with heat-killed S bacteria (that is, S bacteria that had been heated to high temperatures, causing the cells to die), the heat-killed S bacteria did not cause disease in mice
When harmless R bacteria were combined with harmless heat-killed S bacteria and injected into a mouse, not only did the mouse developed disease and died, but when Griffith took a blood sample from the dead mouse, he found that it contained living S bacteria
Griffith concluded that the R-strain bacteria must have taken up what he called a transforming principle from the heat-killed S bacteria, which allowed them to transform into smooth-coated bacteria and become virulent
2) Experiment done by Avery:
Avery, McCarty and MacLeod set out to identify Griffith's transforming principle
They began with large cultures of heat-killed S cells and, through a long series of biochemical steps progressively purified the transforming principle by washing away, separating out, or enzymatically destroying the other cellular components
These results all pointed to DNA as the likely transforming principle but Avery was cautious in interpreting his results
He realized that it was still possible that some contaminating substance present in small amounts, not DNA, was the actual transforming principle
3) Experiment done by Hershey and Chase:
Hershey and Chase studied bacteriophage, or viruses that attack bacteria
The phages they used were simple particles composed of protein and DNA, with the outer structures made of protein and the inner core consisting of DNA
Hershey and Chase concluded that DNA, not protein, was injected into host cells and made up the genetic material of the phage
Answer: In a predator prey relationship, the predator kills and eats the prey. In a parasite/ host relationship, the parisite is feeding off the host, without killing it. If the host dies, the parasite will as well.
