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Friedrich Miescher extracted an "unknown substance" from the nuclei of white blood cells. She found out this substance was so different from protein. This turned out like his biggest discovery. -
Pheovus Levene determined the components of DNA (adenine, guanine, thymine, cytosine, deoxyribose phosphate). However, he did not get the structure right. He said DNA was structured in tetrades. It was too simple to be accurate. (1910) -
Griffith's experiment was the first one to demonstrate that DNA carries genetic material through bacteria transformation. But, how did he do it?
He studied two strains of Streptococcus pneumoniae (as he was bacteriologist):
type Smooth (virulent) -- mouse dies
type Rough (non-virulent) -- mouse lives
heat-killed smooth -- mouse lives
heat-killed smooth + rough -- mouse dies
The strain was transformed somehow... -
This three were "in charge" of finishing and understanding Griffith's results. They did exactly the same as Griffith, but they added one thing: proteases (breaks down proteins) and DNases (breaks down DNA). Here's what they did.
They mixed the previous mix (heat-killed S + R) one with protease and the other with DNase. They injected it to the mice and the one which had protease ended up killing the mouse. BUT! The one who had incorporated DNase made the mouse live.
We know that DNA transforms. -
Chargaff's rules affirmed the balance between "the four pieces of Lego that make everything" (the four bases in DNA). Adenine and Thymine were always in balance, as well as Cytosine and Guanine. Watson and Crick were told about this in 1952. -
In 1951, Watson and Crick defined DNA as a double helix with sugars and phosphates in the inside so the nucleases on the outside. In spite of seeming a "a good theory" it had no chemical sense so they discarded it. It turned out as a little "embarrassment" for those white, rich, intelligent men. -
This is the most famous ray-x photo taken by Rosalind. " Is an X-ray based fiber diffraction image of a paracrystalline gel composed of DNA fiber" - Wikipedia.
This photograph had an enormous impact in DNA structure discovery. Photo 51 was the piece of evidence that unlocked the structure of DNA and its 3D vision. This photograph confirmed the two helix theory. -
The Hershey–Chase experiment (1952) used bacteriophages to answer a question: What carries genetic material? DNA or protein?
So here's what they did
Phage DNA was labeled with ³²P and protein with ³⁵S.
Only ³²P (DNA) entered E. coli cells during infection, while ³⁵S (protein) remained outside.
So, in summary, they proved that DNA carries genetic information, not proteins. This was the final evidence of DNA carrying genetic material!
(that's what I understood at least) -
As it was already explained, once that "the big three" got the photo, they managed to correct the double helix and finally getting it.
They realized (by Chargaff's rules) that A pairs with T and G pairs with C. What is more, they are linked together by hydrogen bonds (which are incredibly strong). Another question which was repetitive was "How to find balance between bases and phosphates". They figured out that the bases must go inside the helix and not otherwise so it wouldn't collapse. -
Once photo 51 was taken by Rosalind, she decided to keep it until all her calculations were accurate. However, as we know, Wilkins and her had a tough relation fed by jealousy and sexism. What it led to Wilkins stealing HER photo without her even knowing. After that, Watson, Crick and Wilkins (obviously) published it in 25th April 1953 leaving Rosalind Franklin in their shadow. -
As it was predicted, this discovery (the final form and structure of DNA) was prized with no less than the Nobel Prize for Watson, Crick and Wilkins (as Franklin died 5 years before that, and women were "incapable" of doing such a thing). But, as expected, they didn't even mention Rosalind and how crucial her discovery had been. Without her photograph their work wouldn't had been possible. (probably)