Question

1. Key Concept List the

Answer 1

DNA

Explanation:    

1) Experiment done by Griffith:

• 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