DNA (Deoxyribo Nucleic Acid)

Double helical structure of DNA molecule:

                  DNA is mainly found in the nucleus. A small amount of DNA is also found in mitochondria and   chloroplast. It a macromolecule formed by the end to end polymerization of a large number of repeated units called deoxyribonucleotides or simply nucleotides.

Each nucleotide is formed by cross-linking of three substances:

                                     i.            Deoxyribose Sugar: The DNA molecules contain deoxyribose sugar and hence, it is       called deoxyribo nucleic acid. Deoxyribose is a pentose sugar (with 5-carbon atoms).

                                     ii.            Phosphate: The phosphate in the DNA is present as phosphoric acid (H₃PO₄). Each   phosphate group is joined to a carbon atom 3’ of one deoxyribose sugar and to carbon atom 5’ of another deoxyribose sugar.

                                     iii.            Nitrogenous Bases: The nitrogenous bases are of two types: Purine and Pyrimidine.

a)      Purine: It is a double ringed structure. It contains Adenine (A) and Guanine (G).

b)      Pyrimidine: It is a single ringed structure. It contains Cytosine (C) and Thymine (T).

Nucleotides and nucleosides:

A sugar molecule with nitrogenous base forms a nucleotide. The nucleosides in DNA are called deoxyribonucleosides and nucleotides are called deoxyribonucleotides.

Nitrogenous base + sugar= Nucleoside

Nucleoside + phosphate= Nucleotide

Or

Nitrogenous base + sugar +phosphate= Nucleotide

DNA is a double helix or double stranded structures. By definition of a helix:
Double helix: The structure of DNA with the two strands of DNA spiraling about one other.
The double helix looks something like an immensely long ladder twisted into a helix, or coil. The sides of the "ladder" are formed by a backbone of sugar and phosphate molecules, and the "rungs" consist of nucleotide bases joined weakly in the middle by hydrogen bonds.