Hershey and Chase Experiment

Introduction

• Discussed in Bogobiology video
• Proved DNA as the hereditary molecule in 1952
• Four major groups of biomolecules: carbohydrates, lipids, proteins, nucleic acids

Background

• Early 20th century confusion about genetic material
• Many believed proteins were the genetic material due to their complexity
• Hershey and Chase's experiment was the third major investigation into genetic material

Previous Experiments

1. Frederick Griffith (1928)

   • Discovered bacterial transformation
   • Proved some substance could transfer between bacteria, changing properties
   • Called it the "transformation principle" using mice and pneumococcus bacteria

2. Avery, MacLeod, and McCarty (1944)

   • Isolated transformation principle
   • Identified it as chemically similar to DNA
   • Destroyed proteins, RNA, and DNA to see effects on transformation
   • Disputed results; contamination concerns raised

Hershey and Chase Methodology

• Aimed to determine if proteins or DNA was responsible for inheritance
• Chose the T2 bacteriophage (virus) because:
  • Reproduces quickly
  • Composed only of nucleic acids and proteins

Bacteriophage Characteristics

• Attacks E. coli
• Injects genetic material into host cell
• Empty protein coat left outside (called "protein ghost")

Experimental Design

• Used two groups of bacteriophages, each with one radioactive component
  • Group 1: Marked protein coat with radioactive sulfur-35
  • Group 2: Marked nucleic acids with radioactive phosphorus-32

Steps of the Experiment

1. Infection
   • Allow bacteriophages to infect host cells
2. Blending
   • Blend to separate empty protein coats from the bacteria
   • Creates a mixture called "suspension"
3. Centrifugation
   • Spin mixture to separate heavier bacteria (pellet) from lighter virus particles (supernatant)

Results

• Measured radioactivity in liquid vs. pellet
• Expected results based on which molecule was responsible for inheritance
• Findings:
  • Supernatant more radioactive in protein group
  • Pellet more radioactive in nucleic acid group
• Conclusion: DNA was the molecule of inheritance

Skepticism and Legacy

• Some scientists skeptical about implications for complex organisms
• Ultimately, findings accepted by scientific community
• Alfred Hershey received Nobel Prize in Medicine (1969); Martha Chase excluded

Conclusion

• Hershey and Chase's experiment was crucial in establishing DNA as the hereditary material.