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A nucleotide is an organic molecule consisting of a nitrogenous heterocyclic nucleobase (a purine or a pyrimidine), a pentose sugar (deoxyribose in DNA or ribose in RNA), and a phosphate or polyphosphate group. As a short-hand, the nucleotides that form the rungs of the ladder in the DNA double-helix are often referred to by biologists, as their base pair identity such as adenine (A), see Category:Purines and Category:Pyrimidines.