-
Discovery of Nucleic Acids
Nucleic acids were discovered in 1868, when twenty-four-year-old Swiss physician Friedrich Miescher isolated a new compound from the nuclei of white blood cells. This compound was neither a protein nor lipid nor a carbohydrate; therefore, it was a novel type of biological molecule. -
Discovery of DNA components
Phoebus Levene discovered all the components of the DNA and defined phosphate-sugar-base units called nucleotides. (1900-1910) -
Levene's Tetranucleotide
Levene is known for his tetranucleotide hypothesis which proposed that DNA was made up of equal amounts of adenine, guanine, cytosine, and thymine. Before the later work of Erwin Chargaff, it was widely thought that DNA was organized into repeating tetranucleotides in a way that could not carry genetic information.
Levene proposed that there were four nucleotides per molecule
Said DNA could not store the genetic code because it was chemically far too simple -
Frederick Griffith and his Transformation Experiment
In this experiment, Griffith mixed the living non-virulent bacteria with a heat inactivated virulent form. He subsequently infected mice with this mixture and much to his surprise, the mice developed pnjeumonia and died. Furthermore, he was able to isolate colonies of the virulent strain from these mice. -
Frederick Griffith
Studied the epidemiology and pathology of 2 strains of Streptococcus pneumoniae
In January 1928 reported the first widely accepted demonstrations of bacterial transformation
Griffith used two strains of Streptococcus: Type S: virulent (deadly) Type R: non-virulent (harmless) Observed bacterial transformation but did not understand the mechanism -
Avery, MacLeod and McCarty
Determined the cause of the transformation in Griffith's Experiment
They took live R and heat-treated S and mixed it with one of two enzymes:
a protease (destroys protein)
a DNAse (destroys DNA)
Published in the Journal of Experimental Medicine
Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types: Induction of Transformation by a Deoxyribonucleic Acid Fraction Isolated from Pneumococcus.DNA not protein was responsible for the bacterial transformation -
Journal of experimental medicine
the paper sugested that DNA, rather tha protein, may be the hereditary material of bacteria and perhaps in higher organisms as well. -
Double Hellix?
Watson and Crick wrote a paper where they described DNA as a double helix with sugars and phospates at the center and the nucleobases facing the outside. It was proven incorrect because all the negative charges would make it explode. -
The race begins
The race was on to determine the structure of DNA in cells and to determine how it codes for proteins and how it replicates.
The problem: DNA exists in two forms
A form (dry form)
B form (wet form, as DNA exists in cells) -
Counting nucleobases and Chargaff's rules
Erwin Chargaff started counting nucleobases because he was interested in the percentages of the different ones. He discovered that in every organism that he looked at, the percentage of Adenine and Thymine were really similar while the Cytosine and Guanine percentage were really similar as well. He did not realise the importance of those findings but he shared those discoveries with Watson and Crick and he was then left out of all the recognition of the discovery of DNA. -
Hershey and Chase
hey made an experiment where they used bacteriophages with either radioactive protein capsules or radioactive DNA. They made the phage infect the bacteria and then they used centrifugation to separate the bacterial cells from the supernatant. Doing that, they discovered that when the DNA was radioactive in the phages, the bacterial cells where radioactive, but when the proteins where radioactive the radioactivity stayed in the supernatant. They concluded that DNA was the genetic material. -
Triple Helix?
Linus Pauling and Robert Corey proposed a triple helix structure for DNA. But it was proven wrong, what they probably saw were the two helix but not clearly so they thought there was a third one. -
Actual structure, Noble Prize and Rosalind Franklin
Rosalind took photo 51 of the actual structure of DNA that proved that there were just two helix but placed in the oposite way that Watson and Crick thought. Wilkins took the picture from Rosalind and gave it to them so they could publish it and get the credit as well as finish their papers. The three of them; Watson, Crick and Wilkins, got the Noble prize. Rosalind didn't get it because she was already dead but none of them gave her any credit for all the research she did.