Nucleobases, also known as nitrogenous bases or simply bases, are nitrogen-containing biological compounds that form nucleosides, which in turn are components of nucleotides. These monomers constitute the fundamental building blocks of nucleic acids. The ability of nucleobases to form base pairs and to stack upon one another leads directly to the formation of long-chain helical structures such as ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).
Five nucleobases—adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U)—are considered primary or canonical. They serve as the fundamental units of the genetic code. The bases A, G, C, and T are found in DNA, while A, G, C, and U are found in RNA. Thymine and uracil are nearly identical, except that thymine contains a methyl group that uracil lacks.
Adenine and guanine have a fused-ring structure derived from purine and are therefore called purine bases. The purine nitrogenous bases are characterized by a single amino group (NH₂): at the C6 position in adenine and at the C2 position in guanine. In contrast, cytosine, uracil, and thymine have a single-ring structure derived from pyrimidine and are therefore called pyrimidine bases.
Each base pair in a typical double-helix DNA molecule consists of one purine and one pyrimidine: either adenine paired with thymine (A–T) or cytosine paired with guanine (C–G). These purine–pyrimidine pairs, known as complementary base pairs, connect the two strands of the helix and are often compared to the rungs of a ladder. This specific pairing is partly due to dimensional constraints, as a purine–pyrimidine combination maintains a constant width in the DNA double helix.
The A–T and C–G pairings are stabilized by hydrogen bonds formed between amine and carbonyl groups on the complementary bases. A–T pairs form two hydrogen bonds, while C–G pairs form three hydrogen bonds.
Nucleobases such as adenine, guanine, xanthine, hypoxanthine, purine, 2,6-diaminopurine, and 6,8-diaminopurine may have formed both in outer space and on Earth. |
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