Nucleotide base

Nucleotide bases (also nucleobases, nitrogenous bases) are <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a>-containing biological compounds that form <a href="/facts/Nucleosides/G40crf0F">nucleosides</a>, which, in turn, are components of <a href="/facts/Nucleotide/pQKEwlJI">nucleotides</a>, with all of these <a href="/facts/Monomer/Gw3CltMb">monomers</a> constituting the basic building blocks of <a href="/facts/Nucleic_acids/2tTgcI0v">nucleic acids</a>. The ability of nucleobases to form <a href="/facts/Base_pair/xmPpPQ9W">base pairs</a> and to stack one upon another leads directly to long-chain helical structures such as <a href="/facts/Ribonucleic_acid/HxPeNfNj">ribonucleic acid</a> (RNA) and <a href="/facts/Deoxyribonucleic_acid/nB89Iauz">deoxyribonucleic acid</a> (DNA). Five nucleobases—<a href="/facts/Adenine/PSeDFBnl">adenine</a> (A), <a href="/facts/Cytosine/JW20j33u">cytosine</a> (C), <a href="/facts/Guanine/hoTSpz0x">guanine</a> (G), <a href="/facts/Thymine/A5C8MnLN">thymine</a> (T), and <a href="/facts/Uracil/KUAcYYpQ">uracil</a> (U)—are called primary or canonical. They function as the fundamental units of the <a href="/facts/Genetic_code/zUK0bY1B">genetic code</a>, 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 distinguished by merely the presence or absence of a methyl group on the fifth carbon (C5) of these heterocyclic six-membered rings.
In addition, some viruses have <a href="/facts/2,6-diaminopurine/0Juv7VSl">aminoadenine</a> (Z) instead of adenine. It differs in having an extra <a href="/facts/Amine/JvfYJfP6">amine</a> group, creating a more stable bond to thymine.
Adenine and guanine have a <a href="/facts/Ring_(chemistry)/EVFmy88Q">fused-ring</a> skeletal structure derived of <a href="/facts/Purine/9ogLp0Wc">purine</a>, hence they are called purine bases. The purine nitrogenous bases are characterized by their single <a href="/facts/Amino_group/SN7W44fv">amino group</a> (−NH2), at the C6 carbon in adenine and C2 in guanine. Similarly, the simple-ring structure of cytosine, uracil, and thymine is derived of <a href="/facts/Pyrimidine/uFRSB8Cq">pyrimidine</a>, so those three bases are called the pyrimidine bases.
Each of the base pairs in a typical double-<a href="/facts/Helix/BqC4AKwL">helix</a> 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 <a href="/facts/Complementarity_(molecular_biology)/WQfqGlBJ">base complements</a>, connect the two strands of the helix and are often compared to the rungs of a ladder. Only pairing purine with pyrimidine ensures a constant width for the DNA. The A–T pairing is based on two <a href="/facts/Hydrogen_bond/85V86IIg">hydrogen bonds</a>, while the C–G pairing is based on three. In both cases, the hydrogen bonds are between the <a href="/facts/Amine/JvfYJfP6">amine</a> and <a href="/facts/Carbonyl/KJ71bo3X">carbonyl</a> groups on the complementary bases.
Nucleobases such as adenine, guanine, <a href="/facts/Xanthine/3ARA2QCg">xanthine</a>, <a href="/facts/Hypoxanthine/QIyA0sQr">hypoxanthine</a>, purine, <a href="/facts/2,6-diaminopurine/0Juv7VSl">2,6-diaminopurine</a>, and 6,8-diaminopurine may have formed in outer space as well as on earth.
The origin of the term <a href="/facts/Base_(chemistry)/M9wA7qeM">base</a> reflects these compounds' chemical properties in <a href="/facts/Acid%E2%80%93base_reaction/z8pbL0X8">acid–base reactions</a>, but those properties are not especially important for understanding most of the biological functions of nucleobases.

Nucleotide base open-in-new

Nucleotide base