A nucleic acid strand is a polymer of nucleotides. Nucleic acids are polymers made of nucleotide monomers. Each nucleotide consists of three parts: a nitrogenous base, a pentose sugar, and a phosphate group. The nitrogen bases are rings of carbon and nitrogen that come in two types: purines and pyrimidines. Pyrimidines have a single six-membered ring. There are three different pyrimidines: cytosine (C), thymine (T), and uracil (U). Purines have a six-membered ring joined to a five-membered ring. The two purines are adenine (A) and guanine (G). The pentose joined to the nitrogen base is ribose in nucleotides of RNA and deoxyribose in DNA. The only difference between the sugars is the lack of an oxygen atom on carbon two in deoxyribose. Because the atoms in both the nitrogenous base and the sugar are numbered, the sugar atoms have a prime after the number to distinguish them.
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