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The Discovery of DNA as the Genetic Material

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Genetics and the Search for Genetic Material

Introduction to Genetics

The field of genetics is the study of heredity and the variation of inherited characteristics. Its foundations were laid in the mid-1800s by Gregor Mendel, but the molecular nature of genetic material was not understood until the 20th century. Early geneticists debated whether DNA or protein was responsible for inheritance, as chromosomes contain both molecules.

  • DNA: Composed of four types of nucleotides (subunits).

  • Proteins: Composed of 20 different amino acids, allowing for greater diversity.

DNA double helix structure

Evidence That DNA Is the Genetic Material

Griffith's Transformation Experiment (1928)

Frederick Griffith's experiments with Streptococcus pneumoniae provided the first evidence that a heritable substance could transfer genetic information between organisms. He worked with two bacterial strains:

  • Smooth (S) strain: Pathogenic, has a protective capsule, causes pneumonia in mice.

  • Rough (R) strain: Non-pathogenic, lacks capsule, does not cause disease.

Colony morphology of R and S strains of Streptococcus pneumoniae

Key Experimental Results

  • Injection of living S cells killed mice.

  • Injection of living R cells did not kill mice.

Results of injecting S and R cells into mice

  • Injection of heat-killed S cells did not kill mice.

Heat-killed S cells do not kill mice

  • Injection of a mixture of heat-killed S cells and living R cells killed mice, and living S cells were recovered from the dead mice.

Mixture of heat-killed S and living R cells kills mice and produces living S cells

This phenomenon, where a non-pathogenic strain is converted into a pathogenic one by exposure to cell debris from the pathogenic strain, was termed transformation.

Transformation of R cells into S cells Summary of Griffith's experiment results

Avery, McCarty, and MacLeod's Experiment

In the 1940s, Oswald Avery, Maclyn McCarty, and Colin MacLeod identified DNA as the 'transforming principle.' They fractionated cell components from heat-killed S cells and found that only DNA could transform R cells into S cells. This provided strong evidence that DNA, not protein, was the genetic material.

  • Key Point: Only DNA from S cells induced transformation of R cells.

Hershey and Chase Experiment (1952)

Bacteriophage Structure and Function

A bacteriophage (or phage) is a virus that infects bacteria. It consists of DNA enclosed in a protein coat. When a phage infects a bacterium, it injects its genetic material into the host cell, directing the cell to produce more viruses.

Structure of a bacteriophage Bacteriophages infecting a bacterial cell

Experimental Design

Hershey and Chase used the T2 phage to determine whether DNA or protein is the genetic material. They labeled phage DNA with radioactive phosphorus (32P) and phage protein with radioactive sulfur (35S), exploiting the fact that DNA contains phosphorus but not sulfur, and protein contains sulfur but not phosphorus.

  • Radioactive sulfur (35S): Labels protein only.

  • Radioactive phosphorus (32P): Labels DNA only.

Labeling phage DNA and protein with radioactive isotopes Experimental setup for labeling phage DNA and protein

Steps of the Experiment

  1. Grow phages in media containing either 35S or 32P to label proteins or DNA, respectively.

  2. Allow labeled phages to infect E. coli bacteria.

  3. Agitate the mixture to remove phage coats from the outside of the bacteria.

  4. Centrifuge the mixture to separate bacterial cells (pellet) from the phage coats (supernatant).

  5. Measure radioactivity in the pellet and supernatant to determine which component entered the cells.

Hershey and Chase experiment: labeling and separation Hershey and Chase experiment: agitation and centrifugation Hershey and Chase experiment: results and conclusion

Results and Conclusion

  • Radioactive phosphorus (32P, DNA) was found inside the bacterial cells (pellet).

  • Radioactive sulfur (35S, protein) remained outside the cells (in the supernatant).

  • Conclusion: DNA, not protein, is the genetic material that is injected by phages into bacteria to direct viral replication.

Key Terms and Concepts

  • Pathogenic bacteria: Bacteria that can cause disease in a host.

  • Transformation (in bacteria): The assimilation of external DNA into a cell, resulting in a genetic change.

  • Bacteriophage: A virus that infects bacteria, composed of DNA and a protein coat.

Summary Table: Key Experiments in Identifying DNA as Genetic Material

Experiment

Organism/System

Key Finding

Griffith (1928)

Streptococcus pneumoniae (mice)

Transformation: Heritable substance can transfer pathogenicity

Avery, McCarty, MacLeod (1944)

Streptococcus pneumoniae (in vitro)

DNA is the transforming principle

Hershey & Chase (1952)

Bacteriophage T2, E. coli

DNA, not protein, is the genetic material

Practice Questions

  • Which result from Griffith’s experiment is an example of transformation? Answer: Mouse dies after being injected with a mixture of heat-killed S and living R cells.

  • Into which component would radioactive sulfur be incorporated? Answer: Protein

  • Into which component would radioactive phosphorus be incorporated? Answer: DNA

  • What is meant by pathogenic bacteria? Answer: Bacteria that can cause disease in a host

  • How do we describe transformation in bacteria? Answer: Assimilation of external DNA into a cell

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