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Hershey and Chase experiment
Quick Biochemistry Basics
Overview
The Hershey and Chase experiment, conducted in the early 20th century, aimed to definitively determine whether DNA or protein served as the genetic material. Building on previous research suggesting DNA's role, they used bacteriophages, viruses that infect bacteria. By radioactively labeling the phage's protein coat and its DNA separately, they tracked which component entered the bacteria during infection. Their findings, showing DNA's radioactivity inside the bacteria, provided strong evidence that DNA, not protein, carries genetic information.
How was this?
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Chapters
- In the early 20th century, scientists debated whether proteins or DNA carried genetic information.
- Some experiments, like those by Avery, MacLeod, and McCarty, suggested DNA was the genetic material.
- Hershey and Chase designed an experiment to resolve this uncertainty.
Understanding the nature of genetic material is fundamental to genetics and molecular biology, explaining how traits are inherited and how organisms function.
The prevailing belief before Hershey and Chase was that proteins, with their complex structures, were the likely candidates for genetic material.
- They used bacteriophages, viruses that infect bacteria, as their model system.
- They radioactively labeled the protein coat of the bacteriophage using sulfur-35 (S35).
- They radioactively labeled the DNA core of the bacteriophage using phosphorus-32 (P32).
- These labeled bacteriophages were then allowed to infect bacteria.
This clever labeling strategy allowed them to distinguish between the protein and DNA components of the bacteriophage and track their fate during infection.
One batch of phages had their protein coats tagged with S35, while another batch had their DNA tagged with P32.
- After infection, a blender was used to detach any bacteriophages still attached to the outside of the bacterial cells.
- The mixture was then centrifuged to separate the heavier bacterial cells (pellet) from the lighter phage components (supernatant).
- The radioactivity in the bacterial pellet was measured.
This step was crucial for isolating the bacteria that had been infected and determining which labeled component had entered them.
Centrifugation separated the bacteria, which settled at the bottom of the tube, from the detached phage particles.
- The bacterial pellet showed high radioactivity when the DNA was labeled with P32.
- Little radioactivity was found in the pellet when the protein was labeled with S35.
- This indicated that the DNA, not the protein, entered the bacteria and carried the genetic information.
This experiment provided definitive proof that DNA is the molecule responsible for heredity, fundamentally changing our understanding of life.
The presence of P32 inside the bacteria confirmed that the phage's DNA had been injected into the bacterial cell.
Key takeaways
- The Hershey and Chase experiment used radioactive isotopes to trace the path of DNA and protein during viral infection.
- By labeling DNA with P32 and protein with S35, they could differentiate which molecule entered the host bacterium.
- The experiment demonstrated that DNA, not protein, is the genetic material passed from bacteriophages to bacteria.
- This finding resolved a major debate in biology and solidified DNA's role as the carrier of genetic information.
- The methodology of using radioactive labeling and separation techniques became a foundational tool in molecular biology research.
Key terms
Hershey and Chase experimentBacteriophageGenetic materialDNAProteinRadioactive labelingSulfur-35 (S35)Phosphorus-32 (P32)Centrifugation
Test your understanding
- What was the central question the Hershey and Chase experiment aimed to answer?
- How did Hershey and Chase use radioactive isotopes to distinguish between DNA and protein?
- Why was centrifugation a critical step in the Hershey and Chase experiment?
- What was the significance of finding P32 radioactivity within the bacterial pellet?
- Based on the experiment, why is DNA considered the genetic material and not protein?