Nucleotides

Nucleic acids -- both DNA and RNA -- are unbranched polymers built from repeating monomer units called nucleotides; because of this, nucleic acids are said to have a polynucleotide structure. The scale of this polymerisation is enormous: a single DNA molecule can contain several MILLION nucleotide units, while an RNA molecule typically contains a few THOUSAND. Each individual nucleotide monomer is itself built from exactly three distinct chemical components joined together: a five-membered-ring monosaccharide (a furanose sugar, often just called 'the sugar' component of the nucleotide), a nitrogen-containing base, and a phosphate group. The sugar component differs between the two nucleic acids: in RNA nucleotides the sugar is D-ribose (carrying -OH groups at both its C-2' and C-3' ring positions), while in DNA nucleotides the sugar is 2-deoxy-D-ribose (carrying an -OH only at C-3', with a plain hydrogen instead of an -OH at C-2' -- this single missing oxygen atom is the entire chemical difference between the two sugars, and is exactly what the 'deoxy' in DNA's full name, deoxyribonucleic acid, refers to). Five different nitrogen-containing bases occur across DNA and RNA nucleotides, falling into two structural families: three are single-ring bases derived from the parent heterocycle pyrimidine -- cytosine (C), uracil (U) and thymine (T) -- and two are double-ring (fused bicyclic) bases derived from the parent heterocycle purine -- adenine (A) and guanine (G). Uracil occurs ONLY in RNA nucleotides, and thymine occurs ONLY in DNA nucleotides (this single base substitution -- U in RNA where DNA instead uses T -- is, together with the sugar difference above, one of the two defining chemical differences between the two nucleic acids; see the Remember box: RNA contains D-ribose, A, G, C, U, while DNA contains D-2-deoxyribose, A, G, C, T). A nucleoside is the two-component unit formed by joining just the sugar and the base together: specifically, by joining the sugar's own anomeric carbon (C-1') to a nitrogen atom of the base, with loss of a water molecule. Because sugar-ring positions and base-ring positions both use plain numbers, sugar-ring atom numbers in a nucleoside are conventionally written with a prime symbol (') to keep them clearly distinguished from the base's own (unprimed) numbering. The precise nitrogen atom of the base used in this bond depends on which family the base belongs to: a pyrimidine base (cytosine, uracil or thymine) bonds through the nitrogen at its own ring-position 1 (N-1); a purine base (adenine or guanine) bonds instead through the nitrogen at its own ring-position 9 (N-9) -- in both cases, always to the sugar's C-1'. Finally, a nucleotide is formed from a nucleoside by adding one further component, a phosphate group, esterified onto the sugar's 5'-OH position; because of this, nucleotides are, chemically, simply the 5'-monophosphate esters of the corresponding nucleosides. This full three-p …