DNA is deoxyribonucleic acid. It is found in the chromosomes, in the nucleus of plant and animal cells. However, it is also found in mitochondria of plant and animal cells and in chloroplast of photosynthetic organisms.
James Watson and Francis Crick, proposed a very simple double helix model for the structure of DNA in 1953 based on X-ray diffraction data.
Structure of DNA is described as follows
(a) Primary structure of DNA
(i) Nucleotides are joined by `3. to 5.` phosphodiester bonds, i.e. each `PO_(4)^(-)` group esterifies to the `3.` hydroxyl group of a pentose sugar and to the `5.` hydroxyl group of next pentose sugar.
{ii) Alternate deoxyribose and phosphate residues form the polynucleotide chain.
(b) Secondary Structure of DNA (Double Helix Model)
(i) DNA is a long polymer of deoxyribonucleotides. It is made up of two polynucleotide chains, where the backbone is colatitude by sugar phosphate and the bases project inside.
(ii) The two chains have antiparallel polarity, i.e. `5. to 3.` for one `3. to 5.` for another.
(iii) The bases in two strands are paired through hydrogen bond (H-bonds) forming base pair (bp). Adenine forms two hydrogen bonds with thymine of the opposite strand. Guanine bonds with cytosine by three H-bonds. Due to this, purine always comes opposite to a pyrimidine. This ensures a uniform distance between the two strands.
(iv) The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm and there are roughly 10 bp in each turn. Due to this, the distance between a base in a helix is about 0.34 nm.
{v) The plane of one base pair stacks over the other in double helix. This confers stability to the helical structure in addition to H-bonds.
