Nucleobases

Nucleobases are nitrogen-containing biological compounds that form nucleosides, which in turn are components of nucleotides; all which are monomers that are the basic building blocks of nucleic acids. Often simply called bases, as in the field of genetics, the ability of nucleobases to form base-pairs and to stack one upon another leads directly to long-chain helical structures such as ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).

When a nucleobase forms a glycosidic bond (sugar bond) with a molecule of ribose or deoxyribose the compound is a nucleoside. A nucleoside joined to or more phosphate groups is a nucleotide. Nucleobases are also known a nitrogenous bases.

Use of the word base is historical, referring to the chemical properties of nucleobases in acid-base reactions, but the term is not especially important for understanding most of the biological functions of nucleobases.

The fused ring skeletal structure of adenine (A) and guanine (G) is derived of purine, hence the name purine-bases; similarly, the simple ring structure of cytosine (C), uracil (U), and thymine (T) is derived of pyrimidine, hence the pyrimidine-bases. In a typical spiral-helix DNA, these bases arrange themselves into base pairs—that is, A pairs with T, and C pairs with G—that connect the two strands of the helix, analogous to rungs connecting the two sides of a ladder. The pairing of purines and pyrimidines may result, in part, from dimensional constraints, as this combination enables a geometry of constant width for the DNA spiral helix. The A-T and C-G pairings function to form double or triple hydrogen bonds between the amine and carbonyl groups on the complementary bases.

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