The Discovery of DNA: Unraveling the Contributions of Frederick Miescher, P.A. Levene, Erwin Chargaff, Maurice Wilkins, and Rosalind Franklin

In this Blogpost you will learn about the discovery of the structure of nucleic acids, specifically DNA. It includes the work of several scientists including Frederick Miescher, who first isolated nucleic acid from fish sperm cells, P.A. Levene, who determined the basic structure of nucleic acids and identified the three main components of DNA, Erwin Chargaff, who discovered the complementary base pairing in DNA, and Maurice Wilkins and Rosalind Franklin, who used X-ray crystallography to produce photographs of DNA which revealed its double helix structure. The description also mentions the role of Watson and Crick in deducing the structure of DNA based on the X-ray crystallographic photograph taken by Franklin.

Friederick Miescher

He was a German biochemist. He isolated a substance in 1869. He named the substance as nuclein, because it was located in the nucleus of the fish sperm cells. Nuclein was later on called nucleic acid, as it was acidic.

 

Work of Levene

In 1920 the basic structure of nucleic acids was determined by the biochemist P. A. Levene. He found that DNA contains three main components (i) phosphate group (ii) five carbon sugars (iii) nitrogen containing bases called purine i.e. Adenine, Guanine, and pyrimidine i.e. Thymine, Cytosine. RNA contains Uracil instead of thymine. Levene concluded that DNA and RNA are made of repeating units called nucleotide.

X-Ray Diffraction of DNA

Erwin Chargaff

He revealed that amount of Adenine was exactly matched with the Thymine and Guanine was equal in amounts to Cytosine in DNA i.e. there is always equal proportion of purine and pyrimidine.

 

Maurice Wilkins and Rosalind Franklin

Maurice Wilkins prepared highly oriented DNA fibers for study, using X-ray crystallography. In this technique an X-ray beam is passed through a crystal of the substance being studied. Part of the X-ray beam is scattered (diffracted) as it passes through the crystal. The way in which it scatters depends upon the structure of the molecule. A photographic plate on the other side of the crystal records a pattern of spots representing the intensity of the emergent X-rays. This pattern reveals information about the locations of various atoms in the crystals, which in turn, can be used to determine the three dimensional shapes of molecules.

 

Wilkins prepared DNA fibers. One of his colleague was Rosalind Franklin. She produced X-ray crystallographic photograph of DNA. Watson saw the photographs. The photograph clearly revealed the basic shape of DNA to be helix. On the basis of Watson's later recollection of the photograph, he and Crick deduced that the helix had a uniform diameter of 2nm. The diameter of the helix suggested that it was made up of two polynucleotide strands. The presence of two strands accounts for the now familiar double helix.