Base pairing: Two
are produced by four nucleotide monomers, nucleobases are in blue
. Guanine (G) is paired with cytosine (C) via three
, in red
. Adenine (A) is paired with uracil (U) via two
hydrogen bonds, in red
Purine nucleobases are fused-ring molecules.
Pyrimidine nucleobases are simple ring molecules.
Nucleobases, also known as nitrogenous bases or often simply bases, are nitrogen-containing biological compounds that form
nucleosides, which in turn are components of
nucleotides, with all of these
monomers constituting the basic building blocks of
nucleic acids. 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).
thymine (T), and
uracil (U)—are called primary or canonical. They function as the fundamental units of the
genetic code, with the bases A, G, C, and T being found in DNA while A, G, C, and U are found in RNA. Thymine and uracil are identical excepting that T includes a
methyl group that U lacks.
Adenine and guanine have a
fused-ring skeletal structure derived of
purine, hence they are called purine bases. Similarly, the simple-ring structure of cytosine, uracil, and thymine is derived of
pyrimidine, so those three bases are called the pyrimidine bases. Each of the base pairs in a typical double-
helix DNA comprises a purine and a pyrimidine: either an A paired with a T or a C paired with a G. These purine-pyrimidine pairs, which are called
base complements, connect the two strands of the helix and are often compared to the rungs 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
carbonyl groups on the complementary bases.
In August 2011, a report based on NASA studies of meteorites suggested that nucleobases such as adenine, guanine,
hypoxanthine, purine, 2,6-diaminopurine, and 6,8-diaminopurine may have formed in outer space as well as on earth.
The origin of the term base reflects these compounds' chemical properties in
acid-base reactions, but those properties are not especially important for understanding most of the biological functions of nucleobases.