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He discovered that the genetic material found in the white blood cells nuclei had an acid, and he decided to call that acid nuclein -
Levene discovered that DNA is made of three parts: a sugar, a phosphate and a base. He named this unit a nucleotide, which was correct. But he thought DNA was organized in groups of four nucleotides and too simple to carry genetic information, so he did not see the real importance of his work. -
In 1910, Leven's continued studying tirelessly to learn more about DNA components and structures. So he proposed the tetranucleotide theory, that affirmed that there were four nucleotides per molecule, and also that DNA couldn't contain our genetic code because its chemical composition was too simple to save it. -
Griffith studied two types of pneumonia bacteria: the smooth type and the rough type. He saw that when he mixed heat-killed smooth bacteria with live rough bacteria, the rough ones became deadly. This showed that something could be transferred from one bacterium to another, that is transformation -
These scientists continued Griffith’s work. They tested which molecule caused transformation by destroying proteins or DNA. When proteins were destroyed, transformation still happened. But when DNA was not destroyed. They concluded that DNA, not proteins, was the genetic material in cells. And DNA is the responsible for the transformation cause it carries the information. -
Pauling, a very famous scientist, also tried to build a model of DNA. He proposed a triple helix with the bases outside and phosphates inside. This was incorrect too, because it did not fit with the real chemical structure. Many people at that time were confused because they studied mixed forms of DNA. -
Watson and Crick built their first model of DNA. They placed the sugar and phosphate in the center and the bases on the outside. This was wrong, because the negative charges of phosphates could not stay together inside. Even if the model looked nice, it did not make chemical sense. -
Franklin was an expert in X-ray crystallography. She took very clear pictures of DNA in its wet form, the same as inside cells. Her most famous image, Photo 51, showed an X shape, the clear sign of a double helix. She wanted to finish her calculations before publishing, but her photo was shown to Watson and Crick without her permission. They used her studies to win the novel and they didn't recognize her. -
Chargaff studied the DNA of different organisms. He measured the amounts of the four bases and noticed a pattern: the amount of adenine always equaled thymine, and the amount of cytosine always equaled guanine. These became known as Chargaff’s Rules, and they helped explain how DNA works. -
They worked with viruses called bacteriophages, made of protein and DNA. They used radioactive labels to follow proteins and DNA during infection. They discovered that only DNA entered the bacteria, while proteins stayed outside. This proved that DNA, not protein, was the true genetic material. -
After seeing Franklin’s photo, Watson and Crick understood their first model was wrong. They used her data to build the correct model of DNA: a double helix with sugar and phosphate on the outside, and bases paired inside they paired: A with T, and C with G. This explained Chargaff’s Rules and how DNA stores genetic information.