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

Nucleic acids are long chain macromolecules which are formed by end to end olymerisation of large number of repeated units called nucleotides. Nucleic acids show a wide range of secondary structures. A secondary structure is the set of interactions between bases and sugar phosphate backbone and is responsible for the shape that nucleic acid.
James Watson and Francis Crick proposed a secondary structure of DNA molecules based on the crystallographic studies.
(i) DNA or deoxyribonucleic acid is a helically twisted double-chain polydeoxyribonucleotide macromolecule.
(ii) The two strands of DNA run anti-parallely to each other called as DNA duplex.
(iii) The spiral twisting of DNA has two types of alternate grooves, i.e., major and minor.
(iv) One turn of $360°$ of the spiral has about 10 nucleotides on each strand of DNA, occupying a distance of about 3.4 nm.
(v) The nucleotides within each strand are held together by the phosphodiester bonds between the 5 ' carbon of one nucleotide and the 3 ' carbon of the adjacent nucleotide. These strong covalent bonds holds the sugar/phosphate backbone together.
(vi) The two strands of DNA are held together by weak hydrogen bonds between the nitrogenous bases. These hydrogen bonds are base specific. That is adenine forms 2 hydrogen bonds with thymine CA=T and cytosine forms 3 hydrogen bonds with guanine (C =G).
(vii) As specific and different nitrogen bases occur on two DNAchains, they are said to be complementary, i.e., purine lies opposite to pyrimidine. This purine-pyrimidine pairing also contributes to the thickness of strand, i.e., 2nm, and makes the two chains complementary.