Discovery of DNA

Key Notes: Discovery of DNA as Genetic Material

• Definition: DNA (deoxyribonucleic acid) is the genetic material that determines an organism’s characteristics.

• Early Belief: Scientists initially thought proteins carried genetic information due to their complexity.

Key Experiments Leading to the Discovery of DNA as Genetic Material

1. Griffith’s Experiment (1920s)

   • Studied two bacterial strains: S strain (virulent) and R strain (non-virulent).

   • Heat-killed S strain + live R strain → Mice died.

   • Conclusion: Something in the dead S strain transformed the R strain into a deadly form.

   • Named the process "transformation."

2. Avery-MacLeod-McCarty Experiment (1940s)

   • Identified DNA as the transforming factor.

   • Inactivated proteins & RNA → R strain still transformed.

   • Inactivated DNA → No transformation.

   • Conclusion: DNA is the genetic material, not proteins or RNA.

3. Hershey-Chase Experiment (1950s)

   • Used bacteriophages (viruses with DNA inside a protein coat).

   • Labeled DNA with radioactive phosphorus (P-32) and proteins with radioactive sulfur (S-35).

   • Only radioactive DNA entered bacterial cells, not proteins.

   • Conclusion: DNA carries genetic information, confirming Avery’s findings.

Key Takeaways

• Transformation: Process where genetic material from one organism alters another.

• Scientific skepticism: Many initially doubted DNA’s role due to its simplicity.

• Final confirmation: Hershey & Chase’s bacteriophage experiment sealed the conclusion.

Timeline of discoveries: Griffith → Avery-MacLeod-McCarty → Hershey-Chase → DNA structure