What is the Hereditary Material?

Introduction to Hereditary Material

Hereditary material is the substance within cells that carries genetic information from one generation to the next. Early research focused on identifying whether proteins or DNA served as the genetic material in chromosomes.

• Proteins: Composed of 20 different amino acids, forming a complex 'alphabet' of building blocks.

• DNA (Deoxyribonucleic Acid): Composed of only 4 nucleotide bases as building blocks.

• Because proteins have a more complex structure, many scientists initially believed proteins were the hereditary material.

Historical Experiments Identifying the Genetic Material

Key Experiments

Several landmark experiments established DNA as the hereditary material:

1. Griffith (1928)

2. Avery, MacLeod, and McCarty (1944)

3. Hershey and Chase (1952)

Griffith's Experiment and the Principle of Transformation

Frederick Griffith's experiment with Diplococcus pneumoniae bacteria demonstrated the phenomenon of transformation, where genetic information could be transferred between bacterial strains.

• Type R (Rough, nonvirulent): Did not cause disease in mice (mouse lives).

• Type S (Smooth, virulent): Caused disease in mice (mouse dies).

• Heat-killed Type S: Did not cause disease (mouse lives).

• Mixture of heat-killed Type S and live Type R: Caused disease (mouse dies), and live Type S bacteria were recovered from the dead mouse.

Conclusion: Genetic information from the dead Type S cells was transferred to the live Type R cells, transforming them into virulent Type S cells. This process was termed transformation.

Avery, MacLeod, and McCarty (1944): Identifying the Transforming Substance

These researchers isolated different components from heat-killed Type S cells and tested which could transform Type R cells.

Conclusion: Only DNA was able to transform Type R cells into Type S, indicating that DNA is the hereditary material.

Key Terms

• Transformation: The process by which genetic material from one organism is taken up by another, resulting in a change in genotype and phenotype.

• Hereditary Material: The substance responsible for inheritance of traits, now known to be DNA.

• Virulent: Capable of causing disease.

• Nonvirulent: Not capable of causing disease.

Summary Table: Key Experiments in Identifying DNA as Genetic Material

Example: The transformation of nonvirulent Type R bacteria into virulent Type S bacteria by exposure to DNA from heat-killed Type S cells is a classic demonstration of DNA's role as the hereditary material.