Q.Nucleic acids exhibit secondary structure. Describe through Wetson-Crick Model.

• Nucleic acids (DNA and RNA) are polynucleotides, and like other biological polymers they organise into a higher-order secondary structure rather than existing as a bare, unstructured chain of bases.
• The Watson-Crick model (1953, based on X-ray diffraction data) describes DNA's secondary structure as a right-handed double helix made of two polynucleotide chains coiled around a common axis.
• The two strands run antiparallel to each other (one strand 5'->3', the other 3'->5'), with the sugar-phosphate backbone on the outside of the helix and the nitrogenous bases projecting inward, stacked one above the other at right angles to the helix axis.
• The two strands are held together by hydrogen bonds between complementary base pairs - Adenine pairs with Thymine through two hydrogen bonds, and Guanine pairs with Cytosine through three hydrogen bonds - so that if the sequence of one strand is known, the other strand's sequence is automatically fixed (complementarity). …

A nucleic acid is a polynucleotide, a long chain built by linking together many nucleotides, each one a nitrogenous base joined to a pentose sugar and a phosphate group. Just as a simple linear sequence of monomers is only the primary level of organisation for a polymer, nucleic acids are also organised into a higher-order, secondary structure - and for DNA, that secondary structure is precisely what the Watson-Crick model, proposed in 1953 on the strength of Wilkins and Franklin's X-ray diffraction data, describes.

According to the model, a DNA molecule is not one lone polynucleotide chain but two chains, coiled together around a common axis in a right-handed double helix. The two strands are antiparallel: one strand runs in the 5' to 3' direction while its partner runs 3' to 5'. The sugar-phosphate backbone of each strand faces outward, forming the outside of the helix, while the nitrogenous bases point inward, stacked flat, one above the other, roughly perpendicular to the helix's long axis. …

Method: Build the Double Helix From Its Named Features

A "describe the model" question is best answered by systematically listing the model's own defining features, in the order a diagram of it would show them, rather than describing the general idea of "two strands together." Before writing anything, mentally check off each of these distinct features so none get skipped: (1) how many strands, and their handedness/coiling; (2) their directionality relative to each other; (3) what lies on the outside of the helix versus the inside; (4) the pairing rule between bases, and how many hydrogen bonds each pair uses; (5) the physical measurements of the helix (diameter, rise per base pair, pitch/bases-per-turn). …