Question

How did Hershey and Chase differentiate between DNA and protein in their experiment while proving that DNA is the genetic material ?

Hint:

Pellet = Bacteria (Inside).
Supernatant = Viral Coats (Outside).
If the radioactivity is in the pellet, the substance entered the host.

Answer 1

Step 1: Understanding the Concept:
By 1952, scientists knew that chromosomes carried genetic information, but they debated whether that information was stored in DNA or Protein.
Hershey and Chase used bacteriophages (viruses that infect bacteria) because they consist only of a DNA core surrounded by a protein coat.
Step 2: Key Formula or Approach:
The experimental approach relied on labeling the two components with specific radioactive tracers that do not overlap.
- DNA contains Phosphorus (\(P\)) in its phosphate backbone but lacks Sulfur (\(S\)).
- Proteins contain Sulfur (\(S\)) in amino acids like Cysteine and Methionine but lack Phosphorus (\(P\)).
Step 3: Detailed Explanation:
The experiment was conducted in three logical stages:
1. Infection:
- Batch 1: Phages were grown with \(^{32}P\). Their DNA became radioactive.
- Batch 2: Phages were grown with \(^{35}S\). Their protein coats became radioactive.
- These phages were allowed to infect E. coli bacteria.
2. Blending:
- The mixture was agitated in a blender. This mechanical force sheared off the viral "ghosts" (empty protein shells) from the surface of the bacteria without killing the bacteria.
3. Centrifugation:
- The mixture was spun at high speed. The heavy bacteria formed a "pellet" at the bottom, while the lighter viral parts remained in the "supernatant" (liquid) at the top.
4. Results:
- In the \(^{32}P\) batch, radioactivity was found in the pellet (inside the bacteria).
- In the \(^{35}S\) batch, radioactivity was found in the supernatant (outside the bacteria).
Step 4: Final Answer:
This proved that DNA was the material that actually entered the cell and directed the production of new viruses, confirming DNA as the genetic material.