MacLeod, McCarty and Avery determined that R strain bacteria transforming into S strain bacteria because of :

Options

A. RNA
B. Protein
C. DNA
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding the Experiment**: In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty conducted experiments to determine the substance responsible for the transformation of R strain bacteria into S strain bacteria.

2. **Background of Griffith's Experiment**: Their work was based on Frederick Griffith's earlier experiment where he discovered that non-virulent R strain bacteria could be transformed into virulent S strain bacteria when exposed to heat-killed S strain bacteria.

3. **Isolation of Cell Extracts**: Avery and his colleagues isolated cell extracts from the virulent S strain bacteria. They aimed to identify the "transforming principle" responsible for this transformation.

4. **Treatment with Enzymes**: The isolated cell extracts were treated separately with two different enzymes:
- **Pancreatic Deoxyribonuclease (DNase)**: This enzyme breaks down DNA.
- **Protease**: This enzyme breaks down proteins.

5. **Mixing with R Strain**: After treatment, the extracts were mixed with non-virulent R strain bacteria and incubated.

6. **Observation of Results**:
- In the extract treated with protease, the R strain bacteria were transformed into virulent S strain bacteria.
- In the extract treated with DNase, the transformation did not occur, meaning the R strain did not become virulent.

7. **Conclusion**: The results indicated that DNA was the transforming principle responsible for the transformation of R strain into S strain bacteria. Since the transformation did not occur in the presence of DNase, it proved that DNA (deoxyribonucleic acid) is the genetic material responsible for this transformation.

8. **Final Answer**: Therefore, the correct answer to the question is **DNA (deoxyribonucleic acid)**.