Give me the experimental proof of transforming principle ?

  

The experimental proof of the transforming principle was a pivotal discovery in molecular biology, demonstrating that DNA, not protein as previously thought is the genetic material responsible for transmitting hereditary information. The key experiments that provided proof of the transforming principle were conducted by Oswald Avery, Colin MacLeod, and Maclyn McCarty in 1944.

Experimental Proof of the Transforming Principle

1. Background

• Before Avery, MacLeod, and McCarty's experiments, it was widely believed that proteins were the carriers of genetic information.
• Frederick Griffith's experiments in 1928 with Streptococcus pneumoniae bacteria showed that a "transforming principle" from heat-killed virulent strains could transfer virulence to non-virulent strains, but the nature of this transforming principle was unknown.

2. Experimental Design

• Bacterial Strains: They used two strains of Streptococcus pneumoniae:
• Rough (R) strain: Non-virulent, lacks a polysaccharide capsule.
• Smooth (S) strain: Virulent, has a polysaccharide capsule.
• Procedure:

1. Heat-Killed S Strain:
• They heat-killed the virulent S strain of bacteria, ensuring that all bacteria were dead but that the DNA (or potential transforming principle) remained intact.
2. Mixing with R Strain:
• They mixed the heat-killed S strain with live R strain bacteria and injected them into mice.
3. Observation:
• Mice injected with the mixture died, and upon autopsy, they found live virulent S strain bacteria in the mice.
4. Control Experiments:
• They conducted control experiments to ensure that the results were not due to any contaminating factors.

3. Results and Conclusion

• The experiments demonstrated that the heat-killed S strain bacteria transferred their ability to produce capsules (virulence) to the live R strain bacteria.
• This transfer of genetic material from the dead S strain to the live R strain indicated that the genetic material responsible for virulence was not destroyed by heat, suggesting it was not protein-based.

4. Follow-Up Experiments

• Enzymatic Treatment: They used enzymes to selectively degrade different biomolecules (proteins, RNA, DNA) from the heat-killed S strain.
• Outcome: Only when DNA-degrading enzymes were used did the transforming ability of the S strain disappear, confirming that DNA was the transforming principle.

5. Publication and Impact

• In 1944, Avery, MacLeod, and McCarty published their findings in the journal Journal of Experimental Medicine, titled "Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types: Induction of Transformation by a Desoxyribonucleic Acid Fraction Isolated from Pneumococcus Type III."
• This paper established that DNA is the genetic material responsible for carrying and transmitting hereditary information, challenging the prevailing view that proteins were the carriers of genetic information.

Conclusion

The experimental proof of the transforming principle by Avery, MacLeod, and McCarty demonstrated through a series of meticulous experiments that DNA is the molecule responsible for transmitting genetic information. This discovery laid the foundation for modern molecular genetics and our understanding of the central role of DNA in heredity and biological processes.