nucleotide
/ NEW-klee-oh-tide /
A nucleotide is the building block of DNA and RNA — the letter of the genetic alphabet, the unit that strings together to spell out a gene. It is also the molecule that carries energy in cells (ATP is a nucleotide). Each one is a small molecular assembly of exactly three parts clicked together: a sugar, a nitrogen-containing base, and a phosphate group.
Take the three parts in turn. The sugar is a five-carbon monosaccharide — ribose in RNA, deoxyribose (one oxygen poorer) in DNA — in its ring (furanose) form. A nitrogenous base, a flat nitrogen-rich ring (adenine, guanine, cytosine, thymine in DNA, uracil in RNA), is attached to the sugar's anomeric carbon through a glycosidic bond, giving a nucleoside. Add a phosphate group, esterified to one of the sugar's hydroxyls, and the nucleoside becomes a nucleotide. To build the DNA or RNA strand, the phosphate of one nucleotide forms a second ester to the sugar of the next, over and over; the result is a backbone that alternates sugar-phosphate-sugar-phosphate, with the bases sticking out sideways like teeth on a comb. These phosphate ester links are the backbone of nucleic acids.
Nucleotides matter because that sticking-out base is where the information lives. In double-stranded DNA, a base on one strand pairs with a complementary base on the other through hydrogen bonds (A with T, G with C), zipping two backbones into the famous double helix. The order of bases along the backbone is the genetic code. Notice how little new chemistry this requires: a sugar in its ring form, a glycosidic bond, and phosphate esters — all functional-group reactions you already know — assembled into the molecule that stores the instructions for life.
Adenosine monophosphate (AMP) is one nucleotide: ribose (sugar) + adenine (base, joined by a glycosidic bond) + one phosphate (an ester on the sugar). Add two more phosphates and you have ATP, the cell's energy currency — same three-part design, just a longer phosphate tail.
Sugar + base + phosphate, joined by a glycosidic bond and a phosphate ester — the unit of DNA, RNA and ATP.
Keep the words straight: base alone, base + sugar = nucleoside, base + sugar + phosphate = nucleotide. The genetic 'base' (adenine, etc.) is a different thing from a Bronsted base, though these rings are mildly basic because of their nitrogen lone pairs.