Experiments

Overview of Pneumonia and Bacterial Strains

  • Pneumonia can be caused by various pathogens, primarily classified into:

    • Viral pneumonia

    • Bacterial pneumonia

  • The focus of this discussion is bacterial pneumonia.

Types of Bacterial Strains

  • **Two Different Strains of Bacteria:

    • Rough Strain:** Harmless and non-virulent.

    • Smooth Strain: Virulent and harmful to mice.

Experiment with Bacterial Strains and Mice

  • A researcher conducts an experiment with the two strains:

    • Rough Strain: Injected into a mouse, resulting in the mouse surviving (Feet Down).

    • Smooth Strain: Injected into a mouse, resulting in the mouse dying (Feet Up).

  • This experiment highlights the differences in virulence:

    • Feet Up: Indicates death of the mouse.

    • Feet Down: Indicates the mouse is alive.

Heat-Killed Bacteria Experiment

  • The researcher then heats the smooth strain to kill it, which denatures proteins and DNA.

    • This process is referred to as heat kill.

    • Result after heating: Does not kill the mouse (Feet Down) as the bacteria is rendered non-viable.

Mixing Strains: Transformation

  • The researcher mixes the non-virulent rough strain with the heat-killed smooth strain, then injects it into a mouse:

    • Unusual outcome: The mouse dies (Feet Up).

    • This phenomenon is due to transformation where non-virulent rough bacteria acquire DNA from killed smooth strain, changing its properties to become virulent.

Key Questions and Concepts

  • What is transformation?

    • Transformation refers to the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material (DNA) from its surroundings.

  • What happened to the rough strain?

    • It became virulent through DNA uptake from the heat-killed smooth strain.

Historical Context and Next Steps in Research

  • At the time of this research, significant global events, such as World War II, influenced laboratory conditions.

  • The original researcher, Griffith, faced challenges leading to a halt in further developments.

Avery, McLeod, and McCarty Experimentation

  • Significance: They built upon Griffith's findings to isolate the transforming factor.

  • They included a preliminary step using enzymes to degrade:

    • Proteases: Break down proteins.

    • RNAases: Break down RNA.

    • DNAases: Break down DNA.

  • Results of their experiments revealed:

    • Mice injected with degraded proteins or RNA survived (healthy).

    • Mice injected with degraded DNA did not survive (dead).

  • Conclusion: The transforming factor must be DNA, proving it is the hereditary material.

Hershey and Chase Experimentation

  • Purpose: Further confirm that DNA is the molecule of inheritance.

  • They used bacteriophages: viruses that infect bacteria, specifically E. coli.

  • Viruses consist of:

    • Protein coat

    • DNA

  • Key methodology:

    • Bacteriophages were grown in radioactive media to label DNA with phosphorus and protein with sulfur.

  • Experiment Steps:

    1. Allow bacteriophages to infect E. coli.

    2. Use a blender to separate the phages from bacteria post-infection.

    3. Centrifuge the mixture to separate components based on density.

  • Observations:

    • Radioactivity location determined the injected material.

    • If radioactivity in pellet of E. coli, it confirmed DNA as genetic material.

  • Final Conclusion:

    • DNA is confirmed as hereditary information, directing RNA and protein synthesis.

Significance of Findings

  • The studies by Griffith, Avery, McLeod, McCarty, and Hershey-Chase were foundational in establishing DNA as the vital hereditary molecule, influencing genetics and molecular biology permanently.

  • Their work contributed to understanding how genetic information is transferred and expressed in living organisms.