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Miescher isolated a new substance from the nuclei of white blood cells. It was acidic, so he called it "nuclein". This was the first discovery of DNA, although its function was unknown. -
Levene identified the components of DNA: four bases (A, T, G, C), sugar (deoxyribose), and phosphate. He also named the units nucleotides, but he thought DNA was too simple to carry genetic information. -
Griffith discovered a phenomenon called bacterial transformation. He found that dead harmful (S strain) bacteria could transform harmless (R strain) bacteria into deadly ones. This showed that some substance from the dead bacteria was passed to the live bacteria and gave them new characteristics. He didn’t know it yet, but this “something” was DNA. -
They showed that DNA (not protein) caused the transformation seen by Griffith. They took live R and heat-treated S and mixed it with one of two enzymes:
- a protease (destroys protein)
- a DNAse (destroys DNA), and found that only when DNA was destroyed, the change did not happen. This proved DNA carries genetic information. -
They suggested DNA had a triple helix structure.
This was also incorrect. -
They proposed a first model of DNA with sugars and phosphates in the center and bases outside.
This model was wrong because it didn’t make chemical sense. -
They used viruses with radioactive labels to prove that DNA, not protein, is passed into bacteria.
This experiment confirmed that DNA is the genetic material. -
Rosalind Franklin took an X-ray picture of DNA. The photo showed an X shape, which was strong evidence that DNA has a helical (twisted) structure.
It also gave information about the distance between base pairs and that the phosphate groups are on the outside of the structure. Wilkins took the photo from Franklin’s desk at King’s College and got it to Watson and Crick in Cambridge. This photograph was used by Watson and Crick to construct the correct model of the double helix in 1953. -
Used paper chromatography and UV spectroscopy to examine the abundance of the nucleobases, and he started to notice something. Chargaff discovered that in DNA, the amount of A = T and C = G. This helped future scientists understand how DNA strands pair. -
Using Franklin’s X-ray images, Watson and Crick built the correct model of DNA: a double helix with the bases on the inside and sugar-phosphate on the outside. This discovery explained how DNA replicates and stores genetic information. -
They were awarded for discovering the DNA double helix structure, one of the biggest scientific breakthroughs of the 20th century. During the awards ceremony, none of them mentioned Franklin, who was the one who reported Photo 51.