The double helix of Watson and Crick

For DNA to fulfill its function of transmission of the genetic message, the cell must produce two Identical copies of DNA during cell division, so every cell has a complete set of genetic information. The double helical structure of DNA proposed by Watson and Crick not only accounted for all the chemical and physical data then available on DNA, but also provided an understanding of the fidelity of reproduction of the DNA molecule. The model was based on the following observations  

Chargaff s group (1) had observed that DNA did not contain Ade, Gua, Thy, and Cyt in equal amounts, as was long believed, but that the percentage of bases in DNA from a variety of sources varied widely. But all DNA s had a regularity in common A and T on one hand and G and C on the other hand, always occurred in equal amounts. Furthermore, the amount of A + G equaled the amount of C + T (Table 4.1). 

If DNA was isolated carefully, a viscous solution, indicative of a macromolecule, was obtained. This macromolecule had the hydrodynamic properties of a rigid rod. When the native DNA molecule was heated or treated with alkali or acid, not only did its optical properties change, but its hydrodynamic properties as well, becoming those of a highly flexible random coil (2). 

It should not be forgotten that two other concepts were 

Important factors in the discovery of the double helical structure of DNA Schrodinger s (5) postulate that an aperiodic crystal must be the carrier of hereditary information and the work of Pauling on the structure of proteins, which had led to the discovery of the a helix (6). Crick s (7) theoretical treatment of helical structures and Pauling s triple-stranded DNA model (8) led Watson and Crick to propose the double helical structure of DNA in 1953 (9). This structure, although its derivation was highly Intuitive, has proven to be essentially correct and has been modified only in detail since. 

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