BackDiscovery of DNA as the Hereditary Material
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Hereditary Material: DNA vs. Protein
Introduction to Hereditary Material
Understanding the molecular basis of heredity is fundamental to biology. Early scientists debated whether proteins or DNA served as the genetic material, as both are found in chromosomes.
Proteins: Composed of 20 different amino acids, forming a complex 'alphabet' and diverse structures.
DNA: Composed of only 4 nucleotide bases (adenine, thymine, cytosine, guanine), leading some to initially doubt its informational capacity.
Historical context: The complexity of proteins led many to believe they were the hereditary material, but key experiments shifted this view to DNA.
Key Experiments Identifying DNA as Genetic Material
Griffith's Transformation Experiment (1928)
Frederick Griffith's work with Diplococcus pneumoniae bacteria demonstrated the phenomenon of transformation, suggesting that a 'hereditary principle' could transfer traits between organisms.
Type R (rough, nonvirulent) bacteria: Did not cause disease in mice.
Type S (smooth, virulent) bacteria: Caused fatal infections in mice.
Heat-killed Type S bacteria: Did not cause disease.
Mixture of heat-killed Type S and live Type R: Caused disease, and live Type S bacteria were recovered from the dead mice.
Conclusion: Some 'transforming principle' from the dead Type S bacteria converted Type R into virulent Type S.
Avery, MacLeod, and McCarty Experiment (1944)
Oswald Avery and colleagues sought to identify the chemical nature of the transforming substance observed by Griffith.
They isolated various components (carbohydrates, lipids, proteins, RNA, DNA) from heat-killed Type S cells.
Each component was added to live Type R cells to test for transformation.
Component | Transformation? |
|---|---|
Carbohydrates/Polysaccharides | No |
Lipids | No |
Proteins | No |
RNA | No |
DNA | Yes |
DNase (enzyme that degrades DNA) | No transformation when DNA destroyed |
Conclusion: Only DNA could transform Type R to Type S, indicating DNA is the hereditary material.
Key Terms and Concepts
Transformation: The genetic alteration of a cell by the direct uptake and expression of foreign genetic material.
Hereditary material: The substance responsible for inheritance of traits, now known to be DNA.
DNase: An enzyme that degrades DNA, used to demonstrate DNA's role in transformation.
Example: Griffith's Experiment Summary
Live R + heat-killed S → Mouse dies, live S recovered.
Heat-killed S alone → Mouse lives.
Live R alone → Mouse lives.
Live S alone → Mouse dies.
Additional info: These experiments laid the foundation for later work by Hershey and Chase (1952), who confirmed DNA as the genetic material using bacteriophages.
