Give the historical background of $DNA$.

(N/A) $DNA$ as an acidic substance present in the nucleus was first identified by Friedrich Miescher in $1869$. He named it as 'Nuclein'.
Due to technical limitations in isolating such a long polymer intact,the elucidation of the 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 equal to one.