noun, plural: nucleotides
A nucleotide is regarded as the basic building block of nucleic acid polymers (e.g. DNA and RNA). It is an organic compound made up of three subunits: a nitrogenous base, a five-carbon sugar, and a phosphate group. The sugar component may either be ribose or deoxyribose. The ribose sugar is the sugar component of the nucleotides that make up RNA. The deoxyribose sugar is the sugar component of DNA. The most common nucleotides are divided into purines and pyrimidines based on the structure of the nitrogenous base. In DNA, the purine bases include adenine and guanine while the pyrimidine bases are thymine and cytosine. RNA includes adenine, guanine, cytosine, and uracil instead of thymine (thymine is produced by adding a methyl to uracil).
Aside from serving as precursors of nucleic acids, nucleotides also serve as important cofactors in cellular signaling and metabolism. These cofactors include CoA, flavin adenine dinucleotide (FAD), flavin mononucleotide, adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADP).
Nucleotides should not be confused to nucleosides, which are also a 5-carbon sugar with a nitrogenous base. However, nucleosides do not have a phosphate group. When a nucleoside is bound to a phosphate group, it yields a nucleotide.
Word origin: nucleo- ("nucleus") + -ide (chemical suffix)