# DNA as the Hereditary Material A series of key experiments conducted from the 1920s to the 1950s conclusively demonstrated that DNA, and not protein, is the molecule that carries genetic information. ## Griffith's Experiment (1928): The Discovery of Transformation Fredrick Griffith's experiment with *Streptococcus pneumoniae* bacteria provided the first evidence that some "principle" or substance could transfer genetic traits between organisms. **Bacterial Strains:** - **S-type (Smooth):** Has a polysaccharide capsule, appears smooth, and is *virulent* (causes disease and death in mice). - **R-type (Rough):** Lacks a capsule, appears rough, and is *non-virulent* (harmless). **Experimental Procedure & Results:** | Injected Substance | Result in Mice | Conclusion | | ------------------------------------------------ | -------------- | --------------------------------------------------------------------------- | | Living S-type bacteria | Died | S-type is virulent. | | Living R-type bacteria | Lived | R-type is non-virulent. | | Heat-killed S-type bacteria | Lived | The capsule alone is not enough to cause disease; the bacteria must be alive. | | Living R-type + Heat-killed S-type bacteria | Died | A substance from the dead S-type bacteria transformed the R-type into S-type. | **Conclusion:** Griffith concluded that a "transforming principle" was transferred from the dead S-type bacteria to the living R-type bacteria, giving them the ability to make a capsule and become virulent. **Transformation:** The transfer of genetic material from one organism to another, altering the genetic makeup of the recipient. ## Avery, Macleod, and McCarty's Experiment (1944): Identifying the Transforming Principle This team set out to identify the specific molecule responsible for Griffith's observed transformation. **Procedure:** 1. They created an extract from heat-killed S-type bacteria. 2. This extract was divided into samples, each treated with a different enzyme to destroy a specific type of molecule. - **Protease:** Destroys proteins. - **RNAase:** Destroys RNA. - **DNAase:** Destroys DNA. 3. Each treated sample was mixed with living R-type bacteria to see if transformation would still occur. **Results:** - When treated with protease (protein destroyed), transformation *occurred*. - When treated with RNAase (RNA destroyed), transformation *occurred*. - When treated with DNAase (DNA destroyed), transformation **did not** occur. **Conclusion:** DNA is the "transforming principle" and therefore the hereditary material. ## Hershey and Chase Experiment (1952): The Definitive Proof Alfred Hershey and Martha Chase provided definitive evidence using bacteriophages (viruses that infect bacteria). **Background:** - Bacteriophages consist of a DNA core and an outer protein coat. - They infect bacteria by injecting their genetic material, which then directs the production of new viruses. - The key question was: *What is injected-DNA or protein?* **Experimental Design:** They used radioactive isotopes to label the two components of the virus. - **Batch 1:** Protein coats were labeled with radioactive sulfur ($^{35}$S), as sulfur is found in proteins but not DNA. - **Batch 2:** DNA was labeled with radioactive phosphorus ($^{32}$P), as phosphorus is a key component of DNA but not most proteins. **Procedure:** 1. The two batches of labeled viruses were allowed to infect separate bacterial cultures. 2. The cultures were agitated (blended) to separate the virus particles from the outside of the bacteria. 3. A centrifuge was used to separate the heavier bacteria (in the pellet) from the lighter viral components and liquid medium (in the supernatant). **Results:** - In the $^{35}$S (protein-labeled) experiment, most of the radioactivity was found in the *supernatant* (outside the bacteria). - In the $^{32}$P (DNA-labeled) experiment, most of the radioactivity was found in the *pellet* (inside the bacteria). **Conclusion:** Only the viral DNA enters the host cell during an infection. Therefore, **DNA is the genetic material** responsible for directing the production of new viruses. ## Possible Questions/Answers - **Q:** What is transformation? **A:** The transfer of genetic material from one organism to another, by which the genetic make-up of the recipient cell is altered. For more on how cells interact with their environment, see . - **Q:** What was the "transforming principle" in Griffith's experiment? **A:** While Griffith didn't identify it, Avery, Macleod, and McCarty later proved it was DNA. - **Q:** Why did Hershey and Chase use radioactive sulfur and phosphorus? **A:** They used $^{35}$S to label the protein coat (as sulfur is in protein but not DNA) and $^{32}$P to label the DNA (as phosphorus is in DNA but not protein). This allowed them to track each component separately during the infection process.