Notes: DNA (DeoxyriboNucleic Acid)
a complex molecule (polymer) found in all living things.
In 1928 scientist Frederick Griffith conducted experiments with two related strains of bacteria, one of which has a smooth coat and caused pneumonia in mice and the other with a rough coat that did not cause pneumonia. His procedure was as follows: (see illustration below)
- Injected mice with smooth bacteria; mice died of pneumonia
- Injected other mice with rough bacteria; mice remained healthy
- Heated and killed smooth bacteria, injected mice, mice remained healthy
- Combined heat killed smooth bacteria with live rough bacteria and cultured them for several days
- Injected combination of dead smooth and live rough bacteria into mice; mice died!
- Cells grown from blood of the dead mice had live smooth bacteria
Griffith proposed an explanation: Transformation; some unknown chemical component of the dead cells was used to transform the harmless rough cells into harmful smooth ones;
Griffith's Experiment AnimationNarrated animation of Griffith's experiment
1944 Avery, McCloud and McCarty first purified and then tested all the chemical components of the smooth bacteria with the rough ones to determine what component caused the transformation; the only component that transformed the bacteria was DNA, but little was known about the composition or functions of DNA.
1947 Chargaff performed chemical tests on DNA and determined that there was always the same amount of Adenine as Thymine and there was always the same amount of Cytosine as Guanine. Conclusion: Adenine and Thymine were paired only with each other, while cytosine and Guanine were paired only with each other as well. As a result of his findings, he proposed Chargaff's Rules, known more commonly as 'base pairing rules.'
1952 Hershey & Chase conducted experiments to determine conclusively whether the transforming chemical components were genetic material, DNA or some type of proteins.
Hershey and Chase used a virus, called a bacteriophage, that infects bacteria to get DNA into cells
Since the protein coat of viruses contains sulfur, the viruses were treated with radioactive sulfur that would bind to the protein, but not DNA, and be easily detected with special equipment
Since DNA has phosphate molecules, but not sulfur, the viruses were also treated with radioactive phosphate that would bind to the DNA but not the protein.
After mixing the treated viruses with untreated bacteria, special techniques were used to track the location of the radioactive isotopes of sulfur and phosphate.
Radioactive sulfur was found on the surface of bacterial cells and radioactive phosphate was found inside the bacterial cells.
Hershey and Chase concluded that it was only the genetic material, DNA, that entered cells and caused transformations.For a better understanding of this critical experiment, access the link below. Select the Narrated option. After viewing, go back and try the quiz!
With what they had discovered about the composition of DNA and Franklin's photographs, Watson & Crick developed and proposed the first model for the structure of DNA. The PBS program The Secret of Photo 51 reveals more about this intriguing story.