The 1944 Avery-MacLeod-McCarty experiment definitively identified DNA as the hereditary genetic material, superseding proteins and RNA. This study built upon Griffith's prior work on bacterial transformation.
Step 1: Griffith's Transformational Experiment.
Griffith observed that mixing non-virulent \( R \)-strain bacteria with heat-killed \( S \)-strain bacteria resulted in the transformation of the non-virulent bacteria into virulent ones. However, he could not identify the specific "transforming principle" among proteins, RNA, or DNA.
Step 2: The Avery-MacLeod-McCarty Experiment.
To pinpoint the transforming principle, Avery, MacLeod, and McCarty isolated proteins, RNA, and DNA from heat-killed \( S \)-strain bacteria. They then assessed each macromolecule's ability to transform the non-virulent \( R \)-strain:
* Enzymes targeting proteins (protease), RNA (RNase), and DNA (DNase) were applied to the samples.
* Transformation persisted when proteins and RNA were degraded, indicating they were not the genetic material.
* However, when DNase destroyed DNA, transformation ceased, strongly suggesting DNA was the "transforming principle."
Step 3: Conclusion.
These findings led Avery, MacLeod, and McCarty to conclude that DNA serves as the genetic material responsible for bacterial transformation, thereby establishing DNA as the molecule of heredity.
Therefore, the accurate conclusion is \( \mathbf{DNA \, is \, the \, genetic \, material \, in \, bacteria.} \)