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Scientist: Friedrich Miecher
Nucleic acids were discovered when, in 1869, a twenty-four-year-old Swiss physician called Frederich Miecher isolated the genet material from the white blood cell nuclei. He noted it had an acidic nature and called nuclein. -
Scientist: Pheobeus Levene
After the discovery of Nucleic acids, Phoebus Levene discovered the basic components of DNA: adenine, guanine, thymine, cytosine, deoxyribose, and phosphate, and identified nucleotides as phosphate-sugar-base units. Unfortunately, he incorrectly proposed that DNA was made of repeating tetrads of four nucleotides and believed it was too simple to carry genetic code. His conclusions were both wrong, and died in 1940 before knowing the significance of his discoveries. -
Scientist: Frederik Griffith
The surprising result came when he mixed the harmless R strain with the heat-killed S strain. The mice died, and live S-type bacteria were later found in their bodies. So, there is a transformation that took place. The smooth strain transformed the other one, but how? Griffith showed that transformation had occurred, but he didn’t discover what the transforming substance was. -
Scientist: Frederik Griffith
Frederik Griffith studied two stains of streptococcus that caused pneumonia, and he was the first person, to demonstrate bacteria transformation. Griffith experimented with 2 strains of Streptococcus and injecting them in a mouse: the virulent smooth strain, and the harmless rough strain. When he injected mice with the R strain, the mice lived. Mice injected with the live S strain died. But when he heat-killed the S strain and injected it alone, the mice lived. -
Scientists: Oswald Avery, Colin MacLeod and Maclyn McCarty.
They explained Griffith's results and showed what caused the transformation, taking the live R and the heat-treated S and mix them with two enzymes: A group was mixed with proteases (destroys protein) and other was mixed with a DNase (that destroys DNA).
Griffith’s mixture+proteases=mouse died
Griffith’s mixture+DNases=mouse lived
This showed that DNA was the responsible for the transformation and the hereditary material of bacteria. -
Scientists: James Watson and Francis Crick
This two scientists came up with their first model and they described DNA as a double helix with sugars and phosphates at the centre and the nucleobases facing the outside. This was incorrect. It made absolutely no chemical sense, all this negative charged phosphates on the inside would have been exploded. -
Scientist: Erwin Chagraff
Chagraff, started counting nucleobases and noticed something really strange. He looked at different organisms, and measured the amounts of the four bases. He founded that in any animal the amount of adenine and thymine was similar. The other ones were also in balance. This was fundamental in the discovery of the structure double helix of the DNA. He shared his discovery with Watson and Crick, but was left out of all the big recognition of the discovery of the DNA. -
These scientists had some experiments with phages, viruses that infect bacteria, they are made with DNA and protein. They used a bacterial cell.
Experiment 1: To label the proteins, they used radiolabeled sulfur, so they could track the proteins in the phage. They allowed them to infect, then the phage disengaged. The result was no radioactive material inside the pellet, it was in the supernatant, the fluid outside. So anything that was protein from the phage didn't enter the bacterial cells. -
Hershey and Chase made a second experiment. They labeled the DNA with P32, a radioactive phosphorus, and allowed the phages to infect as before. After centrifuging, all radioactivity was found in the pellet (in the bacteria) not in the fluid. This surprised many, and the scientists were cautious in their conclusion. Hershey and Chase determined that DNA, not protein, was the genetic material, and that the protein mainly served as packaging. -
Scientist: Linus Pauling
At this point, there was a lot of confusion because DNA exists in two forms. The A form, that it was easier to get at, it's dry. And then the B form, which is actually what DNA really looks like inside cells. Linus Pauling came up with a triple helix model, again with the phosphates and the sugar on the inside and the nucleobases on the outside. He was most certainly looking at X-ray crystallography images that were mixtures of both the A and B form. That was incorrect. -
Scientists: Francis Crick, James Watson and Maurice Wilkins and Rosalind Franklin.
Rosalind took many amazing photographs of the B form of DNA. She figured out how to see the wet form, the form that exists in cells. They call it Photo 51. This photo shows clearly the X in the middle, the sign of a double helix. Wilkins got Photo 51 from her desk at King's College and gave it to Watson and Crick. When they saw the image, they immediately realized their 1951 model was backwards or inside out. -
Scientists: Francis Crick, James Watson and Maurice Wilkins and Rosalind Franklin.
After stole photo 51, Watson and Crick built the model based on Rosalind's image, we have the paper that people know where they describe what DNA looks like. Rosalind died at 37 from ovarian cancer, likely due to X-ray exposure. She contributed to DNA research, wartime gas mask development, coal studies, and virus structures but she couldn’t receive a Nobel Prize because it’s only awarded to living individuals.