How did Hershey and Chase differentiate between DNA and protein in their experiment while proving that DNA is the genetic material?
Hershey and Chase differentiated between DNA and protein by using radioactive isotopes. They grew bacteriophages on two different media: one containing radioactive phosphorus and another containing radioactive sulfur. Since DNA contains phosphorus but not sulfur, viruses grown on radioactive phosphorus had radioactive DNA but not radioactive protein. Conversely, viruses grown on radioactive sulfur had radioactive protein but not radioactive DNA.
These radioactive phages were allowed to attach to E. coli bacteria. After infection proceeded, the viral coats were removed from bacteria by agitating them in a blender. The virus particles were then separated from bacteria by spinning them in a centrifuge.
The results showed that bacteria infected with viruses having radioactive DNA became radioactive, indicating that DNA was the material that passed from virus to bacteria. However, bacteria infected with viruses having radioactive proteins were not radioactive, proving that proteins did not enter the bacteria. This confirmed that DNA is the genetic material.
Explanation
The Hershey-Chase experiment (1952) provided unequivocal proof that DNA is the genetic material. The key methodology involved differential radioactive labeling - phosphorus is present in DNA but absent in proteins, while sulfur is present in proteins but absent in DNA. This clever design allowed them to track which molecule actually entered the bacterial cell during viral infection. The blender step removed the viral protein coats (ghosts) from the bacterial surface, and centrifugation separated the lighter virus particles from the heavier bacteria, enabling clear detection of radioactivity in the bacterial pellet.
Solution Steps
Step 1: Grew bacteriophages on medium with radioactive phosphorus (labeled DNA) and separately on medium with radioactive sulfur (labeled protein)
Step 2: Allowed radioactive phages to attach to and infect E. coli bacteria
Step 3: Removed viral coats by agitation in a blender and separated virus particles from bacteria using centrifuge
Step 4: Checked bacteria for radioactivity - only those infected with DNA-labeled viruses showed radioactivity
Step 5: Concluded that DNA enters bacteria and is the genetic material, not protein