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Discovery of Nucleic Acids
Friedrich Miescher isolated the genetic material from white blood cell nuclei. He discovered that it had an acidic nature and he named it nuclein. -
Discovery of DNA components and Levene's Tetranucleotide
Phoebus Levene discovered all the components of DNA:
adenine, guanine, thymine, cytosine and deoxyribose phosphate. He also defined phosphate-sugar-base units called nucleotides. He proposed that there were 4 nucleotides per molecule but he thought it was organized in tetrades but this was incorrect. Levene proposed that there were 4 nucleotides per molecule. He said that DNA could not store the genetic code because it was chemically far too simple wich was also incorrect. -
Griffith's Transformation Experiment
Frederick Griffith studied the epidemiology and pathology of 2 strains of Streptococcus pneumonie. He reported the first widely accepted demonstrations of bacterial transformation. The experiment consisted of using 2 strains of Streptococcus:
Type S: virulent (deadly)
Type R: non-virulent (harmless)
The type S (smooth) colonies make a capsule, wich makes the mice die.
The type R (rough) colonie do not secret a capule and doesn't kill the mice. But Griffith did'nt figured how the mice died. -
Avery, MacLeod and McCarty's experiment
Avery, MacLeod and McCarty determined the cause of transformation in Griffith's Experiment. They took live R and heat-treated S and mixed it with one of two enzimes:
Proteases, wich destroys protein
DNAse wich destroys DNA R+ Heat-killed S+ Protease = Mice dies
R+ Heat-killed S+ DNAse = Mice lives Continuation: Journal of Experimental Medicine. -
Journal of Experimental Medicine
Avery, MacLeod and McCarty determined the cause of the transformation in Griffith's Experiment and they concluded that was DNA not protein was responsible for the bacterial transformation Griffith observed because if you chop up the DNA with DNAse it's not virulent anymore. This was a very important discovery because it suggested that DNA was the hereditary material not protein in living organisms. -
Discovery of the shape of DNA
Watson and Crick described DNA as a double helix with sugars and phosphates at the center and nucleobases facing the outside but this was marked as incorrect. Later Linus Pauling and R Corey proposed a triple helix structure, but it was incorrect. Rosalind Franklin took a photo of the DNA that showed the X in the middle, the sign of a double helix. But Maurice Wilkins got the photo from her desk and showed Watson and Crick, wich build a model of the DNA and published it. -
Chargaff's rules
This number of percentages came to be known as the Chargaff's Rules: The amount of adenine and thymine were always in balance as well as the amount of Cytosine and Guanine. This was a very important discovery but he didn't acually realize the importance of his findings. However, he did share his discovery with Watson and Crick at the Cavendish Lab in Cambridge in 1952 and these two actually knew what it meant and Chargaff was left out of all the big recognition of the discovery of DNA. -
Hershey-Chase Experiments
Chargaff shared his discoveries with Alfred Hershey and Martha Chase. They used a phage cell, which is a virus that infects bacteria and is made of either DNA or sometimes RNA, and the rest proteins, labeled either with radioactive sulfur allowing them to follow the proteins in the phage or radioactive DNA to follow the movement of DNA during infection and they infected the bacteria. Continuation: Conclusions of Harsey-Chase Experiment. -
Conclusions of Harsey-Chase Experiment
Harsey and Chase concluded that DNA, not protein was the genetic material. A protective protein coat was formed around the bacteriophage, but the internal DNA is what conferred its ability to produce progeny inside bacteria. The only real need for the protein is to serve as packaging and cover the genetic material. The race was on to determine the structure of DNA in cells, how it codes for proteins and how it replicates. The problem was that DNA exists in 2 forms:
A form (dry)
B form (wet) -
Counting Nucleobases
He was interested in the percentages of the nucleobases Erwin Chargaff used paper chromatography and UV spectroscopy to examine the abundance of the nucleobases. He looked at different organisms and mesured the amounts of the 4 bases: adenine, thymine, cytosine and guanine.
The percentages of the bases of all organisms were almost the same as we can see in the image of the event. Continuation: Chargaff's rules. -
A Double-Stranded Helix shaped DNA
Now we know that DNA has the shape of a Double-Stranded Helix. The backbone is made of sugar (deoxyribose) and phosphate groups.
Hydrogen bonds between the nucleobases: A-T and G-C.
The sequence of nucleobases codifies the amino acid sequence of a protein.
Strings of base pairs that code for a product are called genes.