Definition of Nucleotide
A nucleotide is an organic molecule which is known to be the building block of the nucleic acid i.e. DNA and RNA. A nucleoside is composed of a pentose sugar and a nitrogenous base. The pentose sugar present in the DNA is deoxyribose and in the RNA is ribose. When a phosphate group gets covalently attached to the nucleoside, a nucleotide is formed.
There exist four different nitrogenous bases named as adenine, cytosine, guanine and thymine. The structure of RNA has uracil in it instead of thymine. The sugar-phosphate backbone is held by the help of phosphodiester linkages and the bases (A, G, C, T) form hydrogen bonding with each other. The nucleotides perform their role in the mechanisms like cell signalling, metabolism and enzymatic reactions.