The building block of DNA, consisting of a base, sugar, and phosphate.A nucleotide is the repeating unit from which DNA and RNA are built, and it is also a molecule with jobs of its own. Each one has three parts: a nitrogen-containing base, a five-carbon sugar, and one or more phosphate groups. In DNA the sugar is deoxyribose and the bases are adenine, guanine, cytosine, and thymine; in RNA the sugar is ribose and thymine is replaced by uracil. The bases are what carry information, because they pair specifically — adenine with thymine (or uracil), guanine with cytosine — and that pairing is the basis of copying and of reading the genetic code. Nucleotides are not only building material: the same molecules, or close relatives, serve as the cell's energy currency, as signalling molecules, and as cofactors in metabolism.
The individual bases were identified in the late nineteenth century by Albrecht Kossel, who received the 1910 Nobel Prize for the work. The structure of DNA, in which nucleotides form two paired strands, was published by James Watson and Francis Crick in 1953, building on the X-ray diffraction data of Rosalind Franklin and Maurice Wilkins.
Nucleotides are joined into a chain when the phosphate group of one links to the sugar of the next, forming a sugar-phosphate backbone with the bases sticking out to the side. The order of the bases along this backbone is the sequence that genes encode. Because each base presents a shape that fits only its partner, a strand automatically attracts a complementary strand, and each strand can serve as a template for rebuilding the other. Outside of nucleic acids, the most familiar nucleotide is adenosine triphosphate, ATP, whose three phosphate groups store energy that is released when the terminal one is removed.
Nucleotide chemistry underlies DNA sequencing, PCR, and antiviral and anticancer drugs, many of which are nucleotide analogues that block the copying of viral or cellular DNA.
Caffeine works partly by mimicking adenosine, a nucleotide-derived signalling molecule, and blocking its receptor in the brain.
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