Which experiment showed that DNA is the transforming principle using purified extracts?

• A. Griffith Transformation Experiment
• B. Avery, MacLeod, and McCarty Show That DNA Is the Transforming Principle
• C. Hershey-Chase Blender Experiment
• D. Watson-Crick Model

Answer: (B)

The key idea is pinpointing what carries genetic information in transformation and proving it with purified material. Avery, MacLeod, and McCarty purified the transforming substance from heat-killed S bacteria and showed that its ability to convert harmless R bacteria into the S form depended on DNA. When they treated the purified extract with DNase, the transformation activity was lost, whereas degrading proteins or RNA did not stop transformation. This demonstrated that the transforming principle is DNA. They further showed that purified DNA alone could transform R cells, cementing DNA as the genetic material responsible for transformation. The other experiments involve different aspects: Griffith’s work demonstrated transformation itself but used crude material and didn’t identify the transforming substance; Hershey–Chase showed DNA is genetic material in phages, not specifically the bacterial transformation with purified extracts; and the Watson–Crick model is a structural description of DNA, not an experiment about transformation.