The DNA discovery

Discovery of nucleic Acids

1869

Friedrich Miescher, in 1869, isolated the genetic material from white blood cell nuclei, then he noted it had an acidic nature and called it nucleic. Nucleic acids are large biomolecules that play essential roles in all cells and viruses. A major function of nucleic acids involves the storage and expression of genomic information.
Here we have four examples of nucleic acids:
deoxyribonucleic acid (DNA) ribonucleic acid (RNA) messenger RNA (mRNA) transfer RNA (tRNA).

Discovery of DNA components

1907

Phoebus Levene (1869-1940), determined the components of DNA in 1907:

- adenine
- guanine
- thymine
- cytosine
- deoxyribose
- phosphate
defined phosphate-sugar-base units called nucleotides. (today)
DNA is made of chemical building blocks called nucleotides. These building blocks are made of three parts: a phosphate group, a sugar group and one of four types of nitrogen bases. To form a strand of DNA, nucleotides are linked into chains, with the phosphate and sugar groups alternating.

Levene's Tetranucleotide

1910

Levene (1869-1940). Levene proposed that there were four nucleotides per molecule, in that form there is an implication that the four bases are present in equal amounts in DNA.
He said DNA could not store the genetic code because it was chemically far too simple. He was wrong on both of his hypothesis, butt he died before knowing that he actually made a huge contribution to the DNA scientific history.

Fretherick Grifith transformation experiment

1929

He was a bacteriologist, and he studied the epidemiology and pathology of 2 strains of Streptococcus pneumonia, he was the first to discover the bacteria transformation, and in January 1928, reported the first widely accepted demonstrations of bacterial transformation. Griffith used two strains of Streptococcus:
Type S (smooth): virulent (deadly)
Type R (rough): non-virulent (harmless) He observed how the bacterial transformation happened, but did not understand the mechanism.

Avery, McLeod and McCarty

1944

They determined the cause of the transformation in Griffith's experiment.
They did the same has Griffin, (take live R and heat-treated S) but mixed it with two different enzymes:
a protease (destroys protein)
a DNase (destroys DNA)
This experiment showed that it was the DNA that was responsible for the transformation. They published it in the Journal of Experimental Medicine in February 1944.

Chargaff's rules

1952

Erwin Chargaff (1905-2002).
He started counting nucleobases, and discovered that no mater what specie he looked into, the:
Amounts of Adenine = Amounts of Thymine
Amounts of Cytosine = Amounts of Guanine
This went in history as Chargaff's rules, and it became a really huge discovery.

Hersey and Chase

1952

Hersey and Chase did an experiment with bacteriophage, two types: Phages with the DNA inside them and phages with radiolabel sulfur in them. Hershey 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.

Pauling's triple helix

1952

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 facing the outside.
This model was quickly shown to be incorrect and in fact it made no chemical sense Then Linus Pauling and Robert Corey proposed a triple helix structure for DNA.
This structure is made up of three separate DNA strands, each oriented on the outside of the helix and the bases on the inside of the helix.

Rosalin Franklin photo 51

May 6, 1952

Rosalin Franklin was the one who took the picture that proved that the DNA actually had two helices, not three, this picture was showed to Watson and Crick, and thanks to this picture they were able to prove that the triple helix theory was actually incorrect, and that the DNA had the nucleobases connecting with each other, not other the way.
She was never recognized for this picture.

Watson and Crick double helix

1953

Eureka
In 1953 Watson and Crick proved the structure of the DNA, thanks to Rosalin's picture, this was a massive discovery, and couldn't be made without the contribution of all the other experiments and little discoveries that were made through the years. They were given the Nobel Prize for the discovery, this caused that some scientist that contribute to achieve this, felt angry for the excluding that Watson and Crick made, by making the discovery its own.