Search For The Hereditary Material

In 1928, Frederick Griffith embarked upon a series of groundbreaking experiments involving the bacteria Streptococcus pneumoniae, which is known to cause pneumonia. This bacterium exhibits two distinct forms: the smooth strain (S) and the rough strain (R). The smooth strain, covered by a polysaccharide capsule, is virulent and capable of causing disease. On the other hand, the rough strain lacks the capsule and is considered non-virulent as it does not induce pneumonia.

Griffith's Experiments:

Injection of living R strain into a mouse: The mouse survived.

Injection of living S strain into a mouse: The mouse died.

Injection of heat-killed S strain into a mouse: The mouse survived.

Injection of a small number of living R strain and a large number of heat-killed S strain into a mouse: The mouse died of pneumonia.

Griffith’s transformation experiments

Upon examining the blood of the deceased mouse, a substantial number of virulent S strain bacteria were discovered. The non-virulent R strain had transformed into the virulent S strain, suggesting that some factor within the deceased cells had transferred genetic material and altered the genetic makeup of the recipient cell.

The Transforming Principle Unveiled:

In 1944, T. Avery and his colleagues Colin Macleod and Maclyn McCarty further explored this phenomenon, focusing on the transforming principle. They isolated pure DNA from Streptococcus pneumoniae and demonstrated that the loss of transforming activity occurred upon the destruction of DNA. This evidence strongly supported the notion that DNA is the genetic material within cells.

Hershey and Chase's work on Phage and E. Coli

Confirmation through Phage Virus Experiments:

In 1952, Alfred Hershey and Martha Chase conducted experiments on the phage virus T2, providing further validation for DNA as the hereditary material. The phage virus reproduces within bacterial cells. By labeling DNA with radioactive phosphorus (32P) and protein with radioactive sulfur (35S), the researchers differentially tagged these components. Two groups of Escherichia coli bacteria were grown separately in media containing the respective radioactive elements. The radioactive substances were incorporated into the bacteria, and subsequently, viruses were grown using these bacteria as hosts. The resulting viruses were tested for radioactivity.

The conclusive findings revealed that only the DNA labeled with 32P entered the bacteria and reproduced, while the protein labeled with 35S did not. This pivotal experiment solidified the notion that DNA serves as the hereditary material.

In summary, through a series of groundbreaking experiments conducted by Griffith, Avery, Hershey, and Chase, the understanding of genetic material took significant leaps forward. It was demonstrated that DNA, rather than protein, is the fundamental carrier of genetic information, playing a critical role in the inheritance and transmission of traits. These seminal studies laid the foundation for further advancements in the field of genetics and revolutionized our understanding of the molecular basis of life.