Four Key Criteria for Genetic Material:
Information: |
Contains instructions to build an organism. |
Replication: |
Capable of accurate copying (DNA replication). |
Transmission: |
Passed from parent to offspring and between cells during division. |
Variation: |
Accounts for differences within and between species. |
Discovery of Genetic Material:
Early Hypotheses (Late 1800s): |
August Weismann and Karl Nägeli proposed a biochemical basis for inheritance. |
Chromosome Insight: |
Chromosomes, composed of proteins and DNA, identified as carriers of genetic information. |
Griffith's Bacterial Transformation Experiments:
Experiment Background: |
Studied Streptococcus pneumoniae:
Type S (smooth, virulent) strains produce a polysaccharide capsule.
Type R (rough, non-virulent) strains lack this capsule. |
Experimental Steps:
Step 1: |
Injected live type R bacteria into a mouse → Mouse survived, no live bacteria found. |
Step 2: |
Injected live type S bacteria into a mouse → Mouse died, live type S bacteria found in blood. |
Step 3: |
Injected heat-killed type S bacteria into a mouse → Mouse survived, no live bacteria found. |
Step 4: |
Mixed heat-killed type S with live type R bacteria → Injected into a mouse → Mouse died, live type S bacteria found in blood. |
Conclusion:
Genetic material from heat-killed type S bacteria transformed live type R bacteria.
This phenomenon was called "transformation" without knowing the biochemical nature of the transforming substance.
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Transformation Concept:
Living type R bacteria transformed into type S, gaining the ability to produce a capsule. |
This transformation indicated transfer of genetic material. |
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Avery, MacLeod, and McCarty
Focus:Investigated bacterial transformation, following up on Griffith's observations to identify the biochemical nature of the genetic material. |
Experimental Approach:
Question: What substance from dead type S bacteria transforms live type R bacteria? |
Purification Process: Purified macromolecules (proteins, DNA, RNA) from type S Streptococcus pneumoniae.
Found only purified DNA could convert type R to type S bacteria initially. |
Detailed Experiment:
Step 1: |
Mixed purified DNA from type S bacteria with type R bacteria.
Allowed DNA uptake by type R bacteria, converting some to type S. |
Step 2: |
Enzyme Treatments :
DNase: Digests DNA.
RNase: Digests RNA.
Protease: Digests proteins. |
Step 3: |
Aggregated type R cells (non-transformed) removed by centrifugation. |
Step 4: |
Type S cells (transformed) remain in the supernatant. |
Step 5: |
Supernatant plated on growth media to observe bacterial colony formation. |
Step 6: |
Control plates (without DNA extract) showed no type S colonies. |
Conclusion:
DNA from type S bacteria alone could convert type R bacteria to type S, proving DNA as the genetic material. |
Elimination of transformation with DNase confirmed DNA's essential role. |
Hershey and Chase Experiment
Researchers: Alfred Hershey and Martha Chase (1952)
Objective: To determine whether DNA or protein is the genetic material in the T2 bacteriophage, a virus that infects E. coli.
Virus Structure Components:
Capsid (phage coat): Made entirely of protein, consisting of a head, sheath, tail fibers, and base plate.
DNA: Found inside the head of the capsid.
Simplicity: Composed of only DNA and proteins.
Experimental Design
Goal: To identify which component, DNA or protein, enters the bacterial cell and directs the synthesis of new viruses. |
Key Insight: T2 phage injects its genetic material into the bacterial cell while the protein coat remains outside.
Methodology
Labeling: |
DNA labeled with 32P (radioactive phosphorus).
Protein labeled with 35S (radioactive sulfur). |
Infection Process: |
E. coli cells are infected with either 32P-labeled phage or 35S-labeled phage. |
Shearing Force: |
Use a blender to detach phage coats from bacterial cells after allowing the phages to inject their genetic material. |
Centrifugation: |
Separate heavier bacterial cells (pellet) from lighter phage coats (supernatant). |
Detection: |
Measure the radioactivity in the pellet and supernatant using a Geiger counter. |
Results
35S (Protein): Majority found in the supernatant.
32P (DNA): Majority found in the bacterial pellet. |
Conclusion:
DNA enters the bacterial cell, not protein. This indicates that DNA is the genetic material responsible for the production of new viruses. |
Significance
Impact: The experiment provided convincing evidence that DNA, not protein, is the genetic material.
Scientific Legacy: This study was crucial in establishing DNA's role in heredity, greatly influencing molecular biology. |
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