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Discovery of Nucleic Acids
Friedrich Miescher is a Swiss biologist who isolated the genetic material from white blood cell nuclei. He saw it had an acidic nature and that's why he called it nuclein. -
Discovery of DNA Components
Phoebus Levene discovered all the components of DNA and he defined phosphate-sugar-base units called nucleotides. -
Levene's Tetranucleotide
Levene thought it was organized in tetrades, he said there were four nucleotides per molecule. This was a simple structure that it was very difficult for it to contain genetics. -
Frederick Griffith
Frederick Griffith was an English bacteriologist who did experiments with Streptococcus pneumoniae, a bacterium that causes pneumonia. He was the first person to demonstrate bacterial transformation. -
Griffith's transformation experiment
Griffith's transformation experiment involves injecting two different strains of bacteria into mice. The smooth strain was virulent (it causes the mice to die) and the other, rough strain, wasn't. He saw that when they inject a mixture of the two strains (smooth heat-killed strain with rough strain) to the mice, it will die. Instead, the rough strain alone and the smooth heat-killed strain alone didn't kill the mice.
This experiment was the first evidence of the concept of transformation. -
Avery, MacLeod and McCarty (part 1)
They did an experiment to determinate what causes transformation. They did the same experiment as Griffith, but they created two different mixes: one group was mixed with protease, that killed the proteins, and another with DNAse, that destroys the DNA. They took the rough strain and the heat-killed smooth strain, and the mice that was injected with protease died, while the one that was injected with DNAse lives. -
Avery, MacLeod and McCarty (part 2)
They realized that the transformation was happened in the DNA. This was a very big discovery, that was published in the February 1944 Journal of Experimental Magazine. They indicate that was DNA and not protein the hereditary material in the bacteria. -
Double Helix?
James Watson and Francis Crick wrote a paper where they describe DNA as a double helix with sugars and phosphates at the center and the nucleobases on the outside. This model was incorrect because all the negative charges would make it explode, and it would have been a mess. -
Counting nucleobases and Chargaff's rules
Erwin Chargaff started counting nucleobases because he was interested in the percentages of the different ones. He saw that in every organism that he looked at, the percentage of Adenine (A) and Thymine (T) were really similar, while the Cytosine (C) and Guanine (G) percentage were really similar also. He didn't realize the importance of those findings, so he shared those discoveries with Watson and Crick, and he was then left out of the recognition of the discovery of DNA. -
Hershey-Chase Experiments
They made an experiment to confirm that DNA is genetic material. In their experiments, Hershey and Chase showed that when bacteriophages, which are composed of DNA and protein, infect bacteria, DNA enters the host bacterial cell, but most of their protein doesn't. Their discoveries all served to prove that DNA is genetic material. -
X-ray Diffraction Image of DNA Photo 51 by Rosalind Franklin
Rosalind took a lot of amazing photographs of t DNA. She figured out how to see the form that exists in cells. Photo 51 is the most famous image that she took. This image shows clearly the X in the middle that is the sing of a double helix. Rosalind wasn't ready to publish this until she had finished all her calculations -
Triple Helix?
Linus Pauling and Robert Corey propose a triple helix structure for DNA, with the phosphates and the sugar on the inside and the nucleobases on the outside. They saw images where they could see something that looks as a triple helix, but they were incorrect. -
Maurice Wilkins, Watson and Crick, Rosalind Franklin and Photo 51
Maurice Wilkins took Photo 51 from Rosalind's desk at King's College and managed to deliver it to Watson and Crick in Cambridge. When they saw the image, they knew their model was upside down. They built the model based on Rosalind's image in order to get all the credits. The first article to be published was the Watson and Crick paper. The second was one of Stokes and Wilkins. And the third was Rosalind paper. -
Actual DNA structure
Nowadays, we know that DNA has two helix, but placed in the opposite way that Watson and Crick thought. -
The Noble Prize in Physiology or Medicine
Rosalind Franklin didn't share the Nobel Prize because she was already dead, and these prices are only given to living people. None of the three, Watson, Crick and Wilkins, recognized Rosalind's work. Many people think that they were the ones who discovered it because their article appeared first.