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In 1869, discovery of Nucleic Acids. Feriedrich Miescher selected the genetic material from white blood cell nuclei. he said it was a had an acidic nature and I call it nuclein -
Phoebus Levene discover the components of DNA, and determines each one:
-adenine, guanine, thymine, cytosine, deoxyribose
phosphate
Also, defined phosphate-sugar-base units called nucleotides -
He thought it was organized in tetrades. Also, said there were four nucleotides per molecule. That this made such a simple structure.
Secondly, DNA could not store the genetic code because it was chemically far too simple -
Frederick Griffith actually as a bacteriologist who studied two strains of streptococcus that caused pneumonia, and in January 1928 he was the first person to demonstrate bacterial transformation -
He used two different strains of bacteria into mice.
First one is Type S: virulent (deadly) --> this is for smooth colonies secrete a capsule and kill mice
Second one, Type R: non-virulent (harmless) --> Rough colonies do not secrete a capsule and do not kill mice.
This experiment is the first example of transformation -
After performing the experiment, they realized that the transformation was occurring in the DNA. This was a very big discovery, which was published in the Journal of Experimental Magazine of February 1944. Finally, they indicate that it was the DNA and not the protein that was the hereditary material in the bacterium. -
The three of them did an experiment to determine what causes the transformation. They did the same experiment as Griffith, but with two different mixes: the first was mixed with protease, which kills proteins, and the second with DNase, which destroys DNA. They took the rough strain and the smooth heat-killed strain, and the mice that were injected with protease died, while the one that got the DNase injection lived.
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Some famous model builders, Watson and Crick, came up with their first model in 1951, and they described the DNA as a double helix with sugars and phosphates at the center, and the nucleobases facing the outside. This form is incorrect because all the negative charges would make it explode, and it would have been a mess. -
He didn't realize the importance of those findings, so he shared those discoveries with Watson and Crick, and then went unrecognized as part of the DNA discovery. -
Erwin Chargaff, start counting nucleobases. He emigrated from the United States during the Nazi era, and he became a professor of biochemistry, in a University of Columbia (Medical School). But now was interested in percentages of the nucleobases, and he started to investigate something strange. . He saw that in each organism he looked at, the percentage of Adenine and Thymine was very similar, but while the percentage of Cytosine (C) and Guanine (G) were also similar.
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Hershey and chase concluded that it was the DNA and not the protein that was the genetic material, and that the only real need for the protein was to sort of serve as packaging, to cover the thing. -
They did an experiment to confirm if DNA was genetic material. In their experiments, Hershey and Chase showed that when bacteriophages, which are made up of DNA and protein, infect bacteria, the DNA enters the host bacterial cell, but most of its protein does not. These discoveries served to demonstrate that DNA was part of the genetic material. -
Rosalind took a lot of photographs, all of them amazing. These were about of the B form of DNA. She gets to see the wet form, the form that exists in cells. This is the most famous image that she got. They call it photo 51, and this photo shows very clearly the x in the middle that is the sing of a double helix. Rosalind was a stickler for detail, and she was not prepared to publish this until she finished all her calculations.
1953 -
Linus Pauling, discovered the structure of alpha helices and beta sheets in proteins, devised a triple helix model, with the phosphates and sugar on the inside and the nucleobases on the outside. He was sure, looking at X-ray crystallography images, that they were mixtures of the A and B forms. That turned out to be incorrect. -
Today, we know that DNA is a double-stranded helix with a backbone made of antiparallel sugar and phosphate groups. Hydrogen bond between the nucleobases, A with T and G with C. That's why they are always in the same amounts.