the primary structure of DNA represents a long polynucleotide chain DNA double helix

 

Although the primary structure of DNA represents a long polynucleotide chain, it does not provide any clue how the essential tasks of a genetic material, i.e., packing and storing information, releasing information in a definite and regulated manner, replicating itself and allowing mutational changes for evolution can be accomplished by this macromolecule. The answer lies to the secondary structure adopted by DNA in the

biological environment to perform each of these functions. Elucidation of the secondary double helical structure of DNA, considered as the most important discovery of biological science goes to the credit of James Watson and Francis Crick, who solved the riddle of life in 1953. The race to the discovery of DNA double helix began in late 1940s, invoLving many reputed scientists of the world. E. Chargaff in 1940s after comparing base composition of many species noted that purine and pyrimidine arc present in equimolar ratio, and concentration of adenine is equal to thyminewhile concentration of guaninc is equal to cytosine in all these

organisms. At about same time, DNA could be made crystalline and X-ray crystallographic analyses were being performed to understand the structure of DNA by Astbury, R. Franklin and M. Wilkins. One of the X-ray photographs of DNA by Franklin, famous as ‘photograph 51’ showed that crystalline DNA is a helical structure with a regular turn and helix diameter of 20 A. Kinetic studies on DNA in solution suggested that DNA might have hydrogen bonds. Based on these information, Watson and Crick proposed the structure of B-DNA, s’hich is a right handed double helix containing two polvnucleotide chains intertwined around each