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Democritus of Abdera, named the building blocks of matter atomos, meaning literally “indivisible,” about 430 bce.
In 1869, a Swiss chemist named Johann Friedrich Miescher first isolated DNA during his research on the component of white blood cells. He noticed that when he added acid to the salt solution of the cell, a substance separated from the solution. This substance then dissolved again when an alkali was added. As a result, he discovered that this substance had unexpected properties different from other proteins. Johann called this mysterious substance 'nuclein' because he believed it had come from the white blood cell's nucleus.
In 1881 Albrecht Kossel identified "nuclein" as nucleic acid and provided its present chemical name, Deoxyribonucleic acid (DNA). He also named the 4 nucleotide bases which are the building blocks of DNA: Adenine, Cytosine, Guanine, Thymine.
In 1944, an immunologist named Oswald Avery discovered if a harmful form of living bacteria was mixed with an inert but lethal form of the pathogen, the harmless bacteria would become deadly. Determined to find out the cause of this transformation, he purified gallons of bacteria and with groundbreaking evidence, he proved that the substance is a form of nucleic acid, with further research it is revealed to be DNA. As a result, he found out that the chemical basis of DNA is the ‘transforming principle’ of living organisms.
After achieving a doctorate in physical chemistry from Cambridge University, Rosalind Franklin spend three years to master the X-Ray diffraction techniques at the Laboratory Central Services in Paris. In 1952 she returned to London to enhance the techniques of X-ray crystallography with Maurice Wilkins. During one of their investigation on the structure of DNA , they were able to produce two sets of high resolution photographs of DNA fibers, known as “Exposure 51”. By using the photographs Franklin calculated the position of the DNA molecules, and proposed that the DNA phosphates backbone forms a non-plan hexagonal shape.
Watson and Crick describe their double-helix model as having an anti-parallel orientation, meaning that it is composed of two strands of DNA backbone progressing in the opposite directions. Therefore, in a double-stranded DNA molecule, the 5' end of one strand always aligns at a straight line with the 3' end of its partner. Furthermore, the geometry of the DNA double helix creates major grooves and narrower grooves that run along the length of the DNA molecule. Those orientations of grooves are responsible for important binding sites of proteins that maintain the stability of DNA and regulate our gene activity. The two strands of the DNA are held together by hydrogen bonds between nitrogenous bases on opposite strands. This structure of the base-pairing explains why the quantity of Adenine always equals that of Thymine, and the number of Cytosine equals that of Guanine. The pairing connection in the double helix also provides the possibility of genetic replication, as each base will always bond to a specific base. As a result, those base-pairs lie flat, forming a "rung" on the ladder of the DNA molecule. Although Watson and Crick's original model proposed that there were two hydrogen bonds between the bases of each pair, we know today that G and C form an additional bond. Nevertheless, the double-helix model was the first to cross the finish line in this Scientific race.
Double helix (original)
Two chains of nucleotides
The double helix structure unlocked doors for our understanding in DNA's functions, and gave us deeper insight about the inner mechanism of living things. This grand discovery had sparked a great leap on genetic studies(e.g genetic modification such as Crispr) thus, marked a milestone in the history of biology. As a result, Watson, Crick and Wilkins received the Noble Prize in 1962 both for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material. However, Rosalind Franklin died before her contributions are fully recognized.