Describe the structure of $DNA$ as proposed by Watson and Crick.

(N/A) The structure of $DNA$ was proposed by James Watson and Francis Crick in $1953$ based on $X$-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin.
Key features of the double helix model include:
$1$. $DNA$ consists of two polynucleotide chains where the backbone is constituted by sugar-phosphate,and the bases project inside.
$2$. The two chains have anti-parallel polarity. If one chain has the polarity $5' \rightarrow 3'$,the other has $3' \rightarrow 5'$.
$3$. The bases in two strands are paired through hydrogen bonds forming base pairs $(bp)$. Adenine $(A)$ forms two hydrogen bonds with Thymine $(T)$,and Guanine $(G)$ forms three hydrogen bonds with Cytosine $(C)$.
$4$. The two chains are coiled in a right-handed fashion. The pitch of the helix is $3.4 \ nm$ $(34 \ \mathring{A})$,and there are roughly $10$ base pairs in each turn.
$5$. The plane of one base pair stacks over the other in a double helix,which confers stability to the helical structure.