A nucleotide is composed of a sugar, a phosphate and a heterocylic nitrogenous base. Nucleotides act as the monomers for nucleic acid polymers, energy carriers within the body and essential coenzymes.
A single nucleotide is referred to as a mononucleotide unit, or MNU. The pentose sugar present in a nucleotide is found in ring form. It can be either D-ribose or 2'-deoxyribose. The form 2'-deoxyribose lacks a hydroxide on the 2' carbon that is present on D-ribose, and it instead contains two hydrogens.
Ribose is found in RNA, and deoxyribose is found in DNA. The phosphate can be composed of one, two or three phosphates. It can be either a phosphate ion or phosphoric acid. The phosphate bonds to either the 3' carbon or 5' carbon of the sugar.
The nitrogenous base present on a nucleotide is planar, aromatic and heterocyclic. It can be a two-ringed purine, such as adenine or guanine, or a one-ringed pyrimidine, such as thymine, uracil and cytosine. Thymine is found only in DNA, while uracil is found only in RNA. One nitrogen present in the base is linked to the sugar molecule through a N-glycosidic bond. The other forms of nitrogen create the hydrogen bonds that allow nucleic acids to form complementary base pairs.