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The double-stranded DNA structure and its four nucleotides. DNA is composed of nucleotide monomers containing a phosphate, a sugar (deoxyribose), and a nucleobase. In DNA, the nucleobase adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). In RNA, thymine (T) is replaced by uracil (U), and the sugar is ribose.
