DNA

Discovery of Nucleic Acids

1869

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.
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. DNA, encodes the information cells need to make proteins.

Discovery of DNA Components

1907

Phoebus Levene determined the components of DNA:

-adenine, guanine, thymine, cytosine, deoxyribose phosphate
He found that DNA was essentially a long-chain molecule, made up of four different nucleotides, ribose sugar, and phosphate.

Levene's Tetranucleotide

1910

Levene is known for his tetranucleotide hypothesis which proposed that DNA was made up of equal amounts of adenine, guanine, cytosine, and thymine. And also he said DNA could not store the genetic code because it was chemically far too simple

First bacterial transformation

1928

Griffith discovered bacterial transformation, a momentous event in the history of genetics, since hereditary determinants could be transferred from one bacterium to another.
Griffith used two strains of Streptococcus:
Type S: virulent (deadly)
Type R: non-virulent (harmless)
He observed bacterial transformation but did not understand the mechanism.

Cause of the bacteria transformation

1944

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)
and they published in the Journal of Experimental Medicine
Finally, after many experiments, they came to the conclusion that DNA was responsible for the bacterial transformation that Griffith observed.

Double Helix

1951

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.

Hershey and Chase experiment

1952

The experiment was carried out to check whether what made the bacteria produce viruses was the DNA or the proteins.
The experiment consisted of putting the radioactive DNA inside the bacteria to check if the genetic material was in the proteins or in the DNA.
The conclusion was that the genetic information was in the DNA.

Rosalind Franklin photo 51

1952

Rosalind Franklin took many photographs of the DNA.
The photo was named photo 51. In a very suspicious way, the photo reached the hands of Maurici Wilkins. When he saw it, he quickly knew the meaning of that photo. What they predicted long ago was true.
The structure of DNA was the double helix.
The picture shows the double-helix structure of deoxyribonucleic acid: the molecule containing the genetic instructions for the development of all living organisms.

Counting nucleobases

1952

Erwin Chargoff counted nucleobases.
He used paper chromatography and UV spectroscopy to examine the abundance of nucleobases and began to notice something very strange.
It was known as "Chargoff's rules"
He found:
Amounts of adenine = Amounts of thymine
Quantities of Cytosine = Quantities of Guanine
This always happened in every species.

Triple Helix

1952

Linus Pauling and Robert Corey proposed a triple helix structure for DNA.
The triple helix has two strands from conventional DNA plus a ribbon that broke off from other DNA.

Watson and Crick's 3D Model of DNA

1953

In 1953, Watson and Crick suggested a double-helix model of DNA structure in the journal Nature. Their double-helix, molecular model of DNA was then based on a single X-ray diffraction image taken by Rosalind Franklin and Raymond Gosling in May 1952.
The Watson and Crick model was published in a series of five articles in the same issue of Nature. In 1962, after Franklin's death, Watson, Crick, and Wilkins jointly received the Nobel Prize in Physiology or Medicine.

Actual model

2023

DNA is 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.