Final answer:
The iteration of the Griffith experiment that conclusively showed transformation was when a combination of live R strain and heat-killed S strain injected into mice resulted in the mice dying and only S strain bacteria being recovered. Avery, MacLeod, and McCarty later confirmed that DNA was the transforming principle.
Step-by-step explanation:
Griffith's Transformation Experiments
Frederick Griffith's experiments, conducted in 1928, were ground-breaking in demonstrating the phenomenon of bacterial transformation. Griffith worked with two strains of Streptococcus pneumoniae: the non-pathogenic rough strain (R) and the pathogenic smooth strain (S). The key iteration that conclusively demonstrated that transformation must have taken place was when Griffith injected mice with a mixture of live R strain and heat-killed S strain, resulting in the mice's death. From the dead mice, only live S strain bacteria were recovered, indicating that some component of the dead S strain had transformed the live R strain into a virulent S strain. This component was identified as the 'transforming principle'.
Avery-MacLeod-McCarty Experiment
Later, in a series of experiments, Avery, MacLeod, and McCarty demonstrated that DNA was the actual 'transforming principle'. They did this by purifying components of the heat-killed S strain bacteria such as DNA, proteins, and lipids, and observed that only DNA was able to transform the non-pathogenic R strain of bacteria into pathogenic S strain bacteria in in vitro conditions.