Describe double helix structure of $DNA$ and how are the sequences of other strand predicted.
 

The base pairing confers a very unique property to the polynucleotide chains.

They are said to be complementary to each other and therefore if the sequence of bases in one strand is known then the sequence in other strand can be predicted.

Also, if each strand from $DNA$ acts as a template for synthesis of a new strand, the two double stranded $DNA$ (daughter $DNA$) thus produced would be identical to the parental $DNA$ molecule. Because of this, the genetic implications of the structure of $DNA$ become very clear.

The salient features of the double helix structure of $DNA$ are as follows:

$(i)$ It is made of two polynucleotide chain where the backbone is constituted by sugar phosphate and bases project inside.

$(ii)$ The two chains have anti-parallel polarity. It means, if one chain has the polarity $5^{\prime} \rightarrow 3$ ' the other has $3^{\prime} \rightarrow 5^{\prime}$

$(iii)$ The bases in two strands are paired through hydrogen bond ($H$-bonds) forming base pairs $[\mathrm{bp}]$

Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa.