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Discovery of Nucleic Acids
Friedrich Miescher which was a Swiss physician and biologist (1844 - 1895) isolated the genetic material from the white blood cell nuclei. He noted it had an acidic nature and called it nuclein. -
Period: to
Discovery of DNA components
During this period of time, Phoebus Levene, a Lithuanian-American Biochemist (1869 - 1940), discovered all of the DNA components.
Determined the components:
- Adenine, guadine, thymine, cytosine, deoxyrubose phosphate Defined phosphate sugar base units called nucleotides -
Levene's Tetranucleotide
Levene did not get the structure of those components. He thought it was organized in tetrades. He proposed this:
- There were four nucleotides per molecule
- DNA could not store the genetic code because it was chemically far too simple
He was incorrect in both of those counts, and he died in 1940, before his work was fully understood. -
Griffith's Transformation Experiment
Frederick Griffith was a bacteriologist (1879 - 1941).He studied the epidemiology and pathology of 2 strains of Streptococcus pneumoniae,he was the first person to demonstrate bacterial transformation.In January 1928 he reported the first widely accepted demonstrations of bacterial transformation.
He used this two strains of Streptococcus:
-TypeS: virulent,deadly
-TypeR: Non-virulent,harmless Trying those out with lab rats, he saw how their bodies reacted differently with each of the strains. -
Avery, MacLeod and McCarty's Experiment
Their experiments explained Frederick Griffith's results.
They determined the cause of the transformation in Griffith's Experiment by taking the live rough and the heat-treated S, but mixing them with one of two enzymes.
One group was mixed with a protease, which destroys protein, and the other was mixed with DNase, which destroys DNA. -
Journal of Experimental Medicine
In February of 1944 the Journal of Experimental Medicine was publised:
"Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types: Induction of Transformation by a Deoxyribonucleic Acid Fraction Isolated from Pneumococcus Type III" They suggest that DNA, rather than protein, may be the hereditary material of bacteria, and perhaps in higher organisms as well. -
Double Helix?
In 1951, Watson and Crick wrote a paper in which they described DNA as a double helix with sugars and phosphates at the center and the nucleobases bacing the outside.
This model was quickly shown to be incorrect and in fact it made no chemical sense. -
Hershey-Chase Experiments (Conclusions)
Hershey and CHase concluded that DNA, not protein, was the genetic material.
A protective coat was formed around the bacteriophage, but the internal DNA is what conferred its ability to produce progeny inside bacteria
Now there a lot of evidence that the DNA is the genetic material. A lot of people tried to discover its structure, but the problem was that DNA exists iun two forms:
- A form (dry form)
- B form (wet form, as it exists in cells) -
Triple Helix?
Linus Pauling, who discovered the structure of the alpha helixes and the beta sheets in proteins, and Robert Corey, proposed a triple helix structure for DNA, with the phosphates and sugar on the inside and the nucleobases on the outside. That turned out to be incorrect -
The structure of DNA's story (Part 1)
These are the characters that form part of this story: Francis Crick, an American Geneticist (1916-2004), James Watson, American Geneticist (1928 -), Maurice Wilkins, New Zealand Physicist (1916-2004) and Rosalind Franklin, British Biophysicist (1920-1958).
Rosalind figured out how to see the wet form of DNA by taking amazing photographs. One of those was called Photo 51, which is the most famous one. When she was still not prepared to publish this, Wilkinns got photo 51 from her desk. Part 2. -
Counting Nucleobases
Erwin Chargaff, an Austrian Biochemist (1905-2002)started counting the nucleobases.He was really interested in percentages so he noticed something really weird.
He looked at different organisms and measured the amounts of the bases (A,T,G,C).In all of the organisms, A and T percentages were really close and the same thing with C and G.Those were Chargaff's rules,there had to be a balance.However, Chargaff didn't realize the importance of this discovery, but he shared it with Watson and Crick. -
Hershey-Chase Experiments (Procedure)
Alfred Hershey(Geneticist & Bacteriologist) and Martha Chase(American Ceneticist) used a bacteria cell and then they took bacteriophages,either with radioactive sulfur to follow the proteins in the phage,or radioactive DNA to follow the movement of DNA.They allowed them to infect the bacteria and then separate what was in the bacteria from what was not, by centrifugation.Then you get the supernatant, the fluid on top,and the compressed bacterial cells.Now all the radioactivity is in the bacteria -
The structure of DNA's story (Part 2)
Wilkins gave the Photo 51 to Watson and Crick in Cambridge. They knew what it meant when seeing the image, so they knew that their model from before was backwards. They built a model based on Photo 51 and got published on some newspapers. -
The Nobel Prize in Physiology or Medicine
"For their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material".
Crick, Watson and Wilkins got this Nobel but the reason why Franklin did not share this is because she had already passed away, and Nobel Prized cannot be given to dead people. Also, neither of the three ever acknowledged the importance of her work in their discovery