Nucleotides are the building blocks of nucleic acids. They are composed of three subunit molecules:
(i) Nitrogenous bases : Cytosine (C). Thymine (T), Adenine (A), Guanine (G).
(ii) Five carbon sugar.
(iii) Phosphate group.
The phosphate group and 5-carbon sugar compose the backbone of the double helix, while the nitrogenous bases are located in the middle and are bonded to each other.