History of DNA Discovery (6): Frederick Griffith's Experiment, Transforming Principle

Anec  > Biology > Genetic material

Frederick Griffith's transformation experiment about bacteria is a turning point in the exploration path of DNA and genetic material. The transforming principle gradually redirected scientists from the erroneous and confused direction (Tetranucleotide hypothesis) back onto the right path.

Griffith’s Experiment

During the Spanish flu pandemic, pneumonia was one of the leading reasons of death. There are two strains of bacteria responsible for pneumonia: the rough (R) strain, which lacks a polysaccharide capsule on its cell wall, and the smooth (S) strain, which has smooth surface due to the polysaccharide capsule. The polysaccharide capsule itself is non-toxic and non-pathogenic, but it enhances its resistance to white cells. The virulent S bacteria bypass the immune system and proliferate massively within the body . R bacteria, on the other hand, are easily defeated by immune cells and are not lethal.

At this time, the British public health officer Frederick Griffith was engaged in research on infectious diseases and the classification of bacteria. He isolated S and R bacteria from the sputum of pneumonia patients. He found that injecting mice with heat-killed S bacteria or R bacteria alone did not lead to death. To his surprise, when he injected mice with R bacteria and heat-killed S bacteria at the same time, the mice not only became infected with pneumonia and died, but also had a large amount of S bacteria isolated from their blood. Before this, biologists believed that bacterial inherited traits (genes) could only be vertically inherited from parents to offspring, and it was not believed that inherited traits (genes) could be transferred horizontally between bacteria.

Frederick Griffith believed that the R bacteria must have obtained some substances from the dead S bacteria, known as the "transforming principle," making them to produce a polysaccharide capsule. Although he discovered the bacteria transformation, he did not delve into what the "transforming principle" was, thus missing the opportunity to discover genetic material and pioneer a brand new field.

The Significance of the Frederick Griffith transformation Experiment

He is remembered not for his contributions to pathogens and infectious diseases but for the Griffith transformation experiment. It points the way for the exploration of genetic material, that is, to identify what the "transforming principle" is. The Griffith experiment directly led to Avery and his team's exploration of the "transforming principle" which was actually nucleic acid.

Frequently Asked Questions

Why does hot water kill bacteria but not denature their DNA?

The three-dimensional structure of bacterial proteins collapses, and proteins lose activity above 60°C. The denature of proteins is usually irreversible. However, DNA is more stable than proteins. Even in boiling water, broken hydrogen bonds only disintegrate the double helix into single strands. When the solution cools, the single strands recombine into a double helix.

Why didn't Frederick Griffith discover the chemical nature of the "transforming principle"?

In Griffith's era, the nucleic acid structure was still dominated by Phoebus Levene's tetranucleotide hypothesis. Most people, including Griffith, scoffed at the idea that nucleic acid was genetic material. They preferred to believe that information about hereditary traits was stored in proteins. Griffith, a public health officer responsible for infectious diseases, naturally used the transforming principle to explain the outbreak of pneumonia in communities.

How do you explain bacteria transformation in Frederick Griffith experiment, from a modern biological viewpoint?

Heat-killed S bacteria left behind various DNA fragments, including genes controlling polysaccharide capsule. These DNA fragments were released from the S bacteria and were taken up by some R bacteria. Then these DNA fragments were integrated into R bacterial genome. Now, R bacteria acquire the ability to produce a polysaccharide protective coating.

Anec  > Biology > Genetic material

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