$DNA$ as an acidic substance present in nucleus was first identified by Friedrich Meischer in $1869 .$ He named it as 'Nuclein'.

Due to technical limitation in isolating such a long polymer intact the elucidation of structure of $DNA$ remained elusive for a very long period of time.

It was only in $1953$ that James Watson and Francis Crick, based on the $X$-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin proposed a very simple but famous Double Helix model for the structure of $DNA$.

One of the hallmarks of their proposition was also based on the observation of Erwin Chargaff that for a double stranded $DNA $ the ratio between Adenine and Thymine and Guanine and cytosine are constant and equals one.