Nucleic acids exhibit secondary structure. Describe through Watson-Crick model.

$\Rightarrow$ In a polypeptide or a protein, amino acids are linked by a peptide bond which is formed when the carboxyl $(-COOH)$ group of one amino acid reacts with the amino $\left(-\mathrm{NH}_{2}\right)$ group of the next amino acid with the elimination of a water moiety (dehydration).

$\Rightarrow$ In a polysaccharide the individual monosaccharides are linked by a glycosidic bond. This bond is also formed by dehydration. This bond is formed between two carbon atoms of two adjacent monosaccharides.

$\Rightarrow$ In a nucleic acid a phosphate moiety links the $3^{\prime}$-carbon of one sugar of one nucleotide to the $5^{\prime}$ carbon of the sugar of the succeeding nucleotide. The bond between the phosphate and hydroxyl group of sugar is an ester bond. As there is one such ester bond on either side, it is called phosphodiester bond.

$\Rightarrow$ Nucleic acids exhibit a wide variety of secondary structures.

$\Rightarrow$ Watson-Crick model of $DNA$.$ DNA $exists as a double helix.

$\Rightarrow$ One full turn of the helical strand would involve $10$ base pairs.

$\Rightarrow$ $DNA $ helical length is $34 AA$. Distance between successive nitrogen base pair is $3.4 AA$.

$\Rightarrow$ Such type of $DNA$ is called $B-DNA$. There are more than a dozen forms of $DNA$ named after English alphabets with unique structural features.