Question

How did Griffith's discovery lead to the conclusion that genes are made of DNA?

Short answer

Griffith's experiment involved injecting mice with different combinations of live and heat-killed Streptococcus pneumoniae strains, revealing that some "transforming factor" was responsible for turning a harmless R-strain bacteria into a virulent S-strain. Further investigation by Avery, MacLeod, and McCarty identified this transforming factor as DNA, providing crucial evidence that genes, which carry hereditary information and determine traits, are made of DNA.

Step-by-step solution

Introduction to DNA and genes

DNA, or deoxyribonucleic acid, is a molecule that carries genetic information necessary for the growth, development, and reproduction of all living organisms. Genes are smaller functional units within DNA that carry specific instructions to make proteins, which perform various tasks within a cell.

Overview of Griffith's experiment

Frederick Griffith was a British bacteriologist who conducted a series of experiments in 1928 using the bacterium Streptococcus pneumoniae. He worked with two strains of the bacteria: a virulent strain (S-strain) that had a smooth polysaccharide capsule (responsible for lethal pneumonia infections) and a non-virulent strain (R-strain) that lacked the capsules and did not cause disease. Griffith aimed to understand if any factors could transform the harmless R-strain into the harmful and virulent S-strain.

Griffith's experimental setups

Griffith performed the following experimental setups on mice: 1. Injected live S-strain bacteria into mice - The mice became ill and died from pneumonia. 2. Injected live R-strain bacteria into mice - The mice did not get sick. 3. Injected heat-killed S-strain bacteria into mice - The mice did not get sick. 4. Injected a mixture of live R-strain and heat-killed S-strain bacteria into mice - Surprisingly, the mice became ill and died.

Interpreting the outcome of Griffith's experiment

The surprising outcome of the fourth setup, where the mice got sick and died after being injected with a mixture of live R-strain and heat-killed S-strain, led Griffith to hypothesize that some "transforming factor" from the heat-killed S-strain bacteria was transferred to the R-strain bacteria, converting the harmless R-strain into a virulent form.

The "Transforming Principle"

Griffith called the mysterious agent responsible for this transformation the "transforming principle." His observation laid the groundwork for further experiments to identify the nature of this transforming factor.

Avery-MacLeod-McCarty experiment

In 1944, a research group consisting of Oswald Avery, Colin MacLeod, and Maclyn McCarty conducted experiments to identify the transforming principle from Griffith's experiment. They purified molecules from the heat-killed S-strains, including proteins, carbohydrates, lipids, and nucleic acids (DNA) and tested each type for their ability to transform the R-strain bacteria. The results showed that only purified DNA from S-strains could transform the R-strain bacteria into the virulent form. This was evidence that the transforming principle was DNA.

Conclusion

Griffith's discovery led scientists to further investigate the nature of the transforming principle, which was eventually identified as DNA by Avery, MacLeod, and McCarty. This experiment provided strong evidence that genes, which are responsible for carrying hereditary information and determining traits, are made of DNA.