Overview:

• Griffith's experiment showed that something caused the transformation of bacteria.
• Avery, MacLeod, and McCarty showed that DNA was responsible, but people were still unconvinced.
• Hershey and Chase finally proved that DNA is the genetic material.
• DNA is a polymer of nucleotides.
• Chargaff's rules
• Watson and Crick summarized the above findings and postulated their model of DNA structure.
• A, B, Z DNA
• RNA structure

Griffith Experiment:

• The bacteria in the experiment had two different strains: R strain (harmless) and S strain (harmful).
• Live R strain had no effect on the mice, while live S strain killed the mice.
• Heat-killed S strain failed to kill the mice.
• When live R strain (harmless) was mixed with heat-killed S strain (now harmless), and the injected mice died.
• CONCLUSION: The heat-killed cells were somehow able to retain and transfer information.

 

Fig 1 . Griffith's experiment discovering a "transforming principle" in heat-killed virulent smooth pneumococcus is that it enables the transformation of rough non-virulent pneumococcus. (This file is licensed under the  Creative Commons Attribution –Share Alike 3.0 Unported license).

Griffith experimented with two different strains of the bacteria Diplococcus pneumoniae: strain R (rough) and strain S (smooth).  The S cells have a protective protein coat which protects them from being destroyed by the host cell's immune system. Therefore, the R strain is harmless while the S strain is harmful. Griffith injected mice with live strain R bacteria. The mice were found healthy and contained no living bacteria. However, when he injected the mice with the S strain, the mice died and Griffith found live S cells in their bodies. He then injected the mice with heat-killed S bacteria. The mice did not die and contained no live bacteria. Nevertheless, when he injected mice with live R cells and heat-killed S cells, the mice died. From this he concluded that the heat-killed cells, although they were not living, still passed their hereditary material to the living R cells somehow.