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Discovery of Nucleic Acids
Nucleic Acids were discovered on 1869 by the swiss biologist and physician Friedrich Miescher. He isolated the generic material from white blood cells. He notes that had an acidic nature, and he called it nuclein. -
Discovery of DNA components
The DNA components were discovered by the Lithuanian-American biochemist Phoebus Levene. He determined the components of the DNA: adenine, guanine, thymine, cytosine and the deoxyribose phosphate. He also defined that there were phosphate-sugar-base units that were called nucleotides. -
Discovery of the DNA components and Levene's Tetranucloide
The biochemist Levene, also proposed the idea that there were four nucleotides on each molecule, and he also said that DNA could not store the genetic code because it was too simple. -
Frederick Griffith
Frederick Griffith was an english baccteriologist who studied the epidemiology and pathology of two strains of a bacteria called "Streptococcus pneumoniae". On the first month of 1928, he reported the first widely accepted demostration of bacterial transformation. -
Griffith's transformation experiment
He used two strains of streptococcus, type S. S is for smooth.These smooth colonies make a capsule, which made the bacteria virulent (deadly).They caused the mice to die.There was also a type R, which is non-virulent (harmless).They don't have that smooth capsule, and mice when exposed to it, they get alive Moreover, when he heated the S strains, mice weren't affected, but when they mixed both and then heat them together, the result was a dead mice. So somehow, the heat transformed the effects -
Avery, MacLeod and McCarty on Griffith's experiments
Avery, McLeod and McCarty (geneticists and physician) worked on the experiments of Griffith trying to give an explanation to what he couldn't They took the live R and the heat-treated S strains and mixed them with 1 of the 2 enzymes:- Proteases (which destroy protein)
- DNase (which destroy DNA)
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Journal of Experimental Medicine
It explains the reasons on why things ocurred in the work that Avery, MacLeod and McCarty made on Griffith's experiment. This journal basically explains that the DNA, rather than protein, may be the hereditary material of bacteria in small organisms, but in bigger as well -
Double Helix
Watson and Crick wroted a paper where they defined the DNA as a double helix with sugars and phosphates in the center and nucleobases in the outside This information was quickly shown to be incorrect. -
Nucleobases
Erwin Chargaff, was an autrian bioquemist who showed a big interest on the percentage of the diferent nucleobases. He was examinating the amount of the bases (adenine, thymine, guanine and cytosine) of different organisms and realized that the levels of Adenine and Thymine are more or less the same an the levels of Guanine and Cytosine too. -
Chargaff's Rules
Chargaff's Rules describe mainly, two things:- The levels of Adenine and Thymine in an organism are always the same
- The levels of Guanine and Cytosine in an organism are always balanced
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Hershey-Chase Experiments (part 1)
On 1952, Alfred Hershey (an american geneticist and bacteriologist), and Marta Chase (an american geneticist) made some experiments on bacteriophages. Bacteriphages are viruses that infect bacteria (they are made of DNA and protein and is composed of a head, a tail and the tail fiber. -
Hershey-Chase Experiments (part 2)
Hershey and Chase used bacteria cells and then they took bacteriophages either labeled with radioactive sulfur (that allowed them to follow the proteins) or radioactive DNA (that allowed them to follow the movement of DNA during the infection) They took both and allowed them to infect the bacteria and then they separated the bacterial cells from the supernatant through centrifugation
After, there was left the supernatant ( everything oustide the cell) and the pellet (compressed bacterial cells) -
Hershey-Chase Experiments (part 3)
To label the proteins they used radio-labeled sulfur (S35). After getting the bacteria being infected they realized that all the S35 was in the supernatant and no S35 was left in the pellet On the second experiment, they labeled the DNA with radioactive phosphorus (P32) and when letting it infect the bacteria, they get exactly the opposite results. All the P32 was in the pellet, and there was not P32 is the supernatant -
Conlusions of Hershey-Chase Experiments
At the end, Hershey and Chase concluded that DNA was the genetic material, and not protein as thought. Out of their experiments they also come up with the information that a protective coat was formed around the bacteriophage, but in fact, is DNA what conferred its ability to produce progeny inside the bacteria. -
Functions and appearence of DNA
The majority of scientists were focusing to determine the structure of the DNA, and how it codes on proteins and its replication. The problem was that the DNA exists in two types of forms:- A form (dry form)
- B form (wet form, as it is in our cells)
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Triple Helix
Linus Pauling (an american biochemist) and Robert Corey proposed a triple helix structure of the DNA by looking at x-ray crystallography images proposingthat the phospate and sugars were also in the center and the nucleobases on the outside. But as the concept of the double helixt it turned out to be incorrect. -
Stealing of Photo 51 and creation of the DNA model
Rosalin Franklin, took a lot of photografies of the B form of the DNA. She figured out how to see the wet form of the DNA. The most famous photo she took is called Photo 51 and it shows clearly a X in the center (sign of a double helix). Maurice Wilkings took that image from her desk and brought it to Watson and Crick. Then, they built a model of DNA based on the Photo 51. -
DNA as a Double-Stranded Helix
With the correct model of DNA made by Watson and Crick we now some things about DNA:- The backbone is made of sugar (deoxyribose) and phosphate groups
- Hydrogens bonds between the nucleobases: Adenine-Thymine and Genine-Cynosite
- The sequence of nucleobases codifies the amino acid sequence of a protein
- Strings of base pairs that code for a product are called genes
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Publishment of the discovery
Three papers were published on the 25th of April edition of the journal "Nature". First, was published the paper of Watson and Crick, the second one was done by Stokes and Wilkings, and the last one was published by Rosalind Franklin and her assistant, Raymond Gosling -
Noel Prize RegonitionB
On 1962, the NoBel Prize was awarded to Francis Crick, Maurice Wilkings and James Watson because of their discoverings about the molecular structure of nucleic acids and its significance for information transfer in living material. Rosalind Franklin wasn't awarded for her discovery because she did already passed away when the Nobel price was awarded.
