Explain the role of the following scientists in the discovery of the structure of $DNA$:
$1.$ Watson and Crick $(1953)$
$2.$ Erwin Chargaff

(N/A) $1.$ Watson and Crick $(1953)$ proposed a very simple but famous $\text{Double Helix}$ model for the structure of $DNA$ based on $X$-ray diffraction data.
$2.$ Erwin Chargaff observed that for a double-stranded $DNA$ molecule,the ratios between $\text{Adenine}$ and $\text{Thymine}$ and between $\text{Guanine}$ and $\text{Cytosine}$ are constant and equal to one,i.e.,$A = T$ and $G = C$.