Frederick Griffith's 1928 experiment demonstrated that DNA is the genetic material through the transformation of bacteria, using virulent (S strain) and non-virulent (R strain) strains of pneumonia. The study showed that when R strain bacteria were mixed with heat-killed S strain bacteria, the R strain transformed into the virulent S strain, leading to the death of the mice. Subsequent biochemical characterization by Oswald Avery and colleagues confirmed that DNA, not proteins or RNA, is responsible for the virulent properties of bacteria.

1. Transforming principle
(Griffith’s Experiment)
DNA- a genetic material
By-Dr R K Upadhyay
PGT, Biology

2. Frederick Griffith
• 1928
• Mice
• Virulent strain of pneumonia
(S strain)
• Non virulent strain of
Pneumonia (R strain)
• Streptococcus pneumoniae)

3. ‘S’ and ‘R’ strain
• Type III-S (smooth) – virulent
• Type II-R (rough)- nonvirulent.
• The III-S strain synthesize a polysaccharide capsule that
protected itself from the host's immune system, resulting
in the death of the host’
• While the II-R strain don’t have that protective capsule
and was defeated by the host's immune system

4. Transformation
• bacteria are capable of transferring genetic
information through a process known
as transformation

5. Griffith experiment
• MICE + S strain------mice died due to Pneumonia
• MICE + R strain------mice alive

6. Griffith experiment
• MICE + heat killed S strain------mice live
• MICE + heat killed S strain + R strain------mice
died due to Pneumonia.

7. Reason
• Due to transformation the genetic material from heat
killed ‘S’ strain transferred to the ‘R’ strain.
• ‘R’ strain (non virulent) transformed into ‘S’ strain
(virunet)

9. Biochemical characterization of transforming principle
• Oswald Avery, Colin MacLeod, and Maclyn McCarty
• Prove DNA is genetic material.

10. Use of enzymes Protease, Rnase, DNase
• MICE + heat killed S strain + R strain +
Protease mice died
• MICE + heat killed S strain + R strain +
RNase mice died
• MICE + heat killed S strain + R strain +
Protease mice died

11. Conclusion
• There is no effect of Proteins and RNA on the
virulent property of Bacteria.
• DNA has virulent property.