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Qualitatively, guanine (G) and cytosine (C) undergo a specific hydrogen bonding with each other, whereas adenine (A) bonds specifically with thymine (T) in DNA and with uracil (U) in RNA Quantitatively, each GC base pair is held together by three hydrogen bonds, while AT and AU base pairs are held together by two hydrogen bonds To emphasize this difference, the base pairings are often represented as "G≡C" versus "A=T" or "A=U"Attached to each sugar ring is a nucleotide base, one of the four bases Adenine(A), Guanine(G), Cytosine(C), and Thymine(T) The first two (A, G) are examples of a purinewhich contains a six atom ring and five atom ring sharing two atoms The second two (C, T) are examples of a pyrimidinewhich is composedO, N, S, and Cglycosidic Bonds Glycosidic bonds are labeled according to the identity of the atom on the second carbohydrate or the functional group The bond formed between the hemiacetal or hemiketal on the first carbohydrate and the h