Discovery of the structure of DNA (article) | Khan Academy
https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-discovery-and-structure/a/discovery-of-the-structure-of-dna
WEBA nucleotide is made up of a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G) or cytosine (C). C and T bases, which have just one ring, are called pyrimidines, while A and G bases, which have two rings, are called purines.
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