Hershey-Chase Experiment
ஹெர்ஷே-சேஸ் பரிசோதனை
Avery, MacLeod, and McCarty’s biochemical analysis left skeptics doubting. Hershey and Chase ended the debate in 1952. If you still think proteins carry genetic information, you are failing the basics of Molecular Biology.
The idea
Hershey and Chase grew bacteriophages in media containing radioactive phosphorus (32P) to label DNA and radioactive sulfur (35S) to label proteins. These phages infected E. coli bacteria. Through blending and centrifugation, they separated the viral coats from the bacterial cells. Only 32P was detected inside the bacteria, proving DNA is the genetic material.
Why it matters
This experiment resolved the long-standing debate over whether DNA or protein was the hereditary material.
NEET lens
NEET focuses on the isotopes used: 32P for DNA and 35S for protein. Remember: 35S is absent in DNA, and 32P is absent in proteins. Do not confuse the centrifugation results; the supernatant contains the viral coat, while the pellet contains the infected bacteria.
Clinical significance
This experiment explains the mechanism of viral pathogenesis. When a virus injects its genome into a host cell, it hijacks the host's machinery to replicate. Understanding this is critical for developing antiviral therapies and gene therapy vectors.
Introduction
Hershey and Chase (1952) gave unequivocal proof that DNA, not protein, is the genetic material, using bacteriophages that infect E. coli.
Core concept
Method: one batch of viruses grown in a medium with radioactive phosphorus (32P) — labels DNA (proteins have no phosphorus); another batch with radioactive sulphur (35S) — labels protein (DNA has no sulphur). The labelled phages infected E. coli; after infection, blending removed viral coats and centrifugation separated the bacteria. Result: bacteria infected with 32P-labelled viruses were radioactive; those with 35S-labelled viruses were not — radioactivity stayed in the supernatant with the protein coats. So DNA entered the bacteria and carried the genetic information.
Go deeper
Criteria for a genetic material: it should replicate, be chemically and structurally stable, provide scope for slow changes (mutation) needed for evolution, and be able to express itself as Mendelian characters. DNA is more stable than RNA; RNA is reactive and catalytic, and was probably the first genetic material.
From Class 10
Viruses inject something into bacteria to make more viruses. This experiment showed that the 'something' is DNA.