H

Cytosine (C)

▲ FIGURE 2-15 Chemical structures of the principal bases in nucleic acids. In nucleic acids and nucleotides, nitrogen 9 of purines and nitrogen 1 of pyrimidines (red) are bonded to the 1' carbon of ribose or deoxyribose. U is only in RNA, and T is only in DNA. Both RNA and DNA contain A, G, and C.

the 1 ' carbon atom of the sugar (ribose or deoxyribose) is attached to the nitrogen at position 9 of a purine (N9) or at position 1 of a pyrimidine (N1). The acidic character of nu-cleotides is due to the phosphate group, which under normal intracellular conditions releases a hydrogen ion (H+), leaving the phosphate negatively charged (see Figure 2-14a). Most nucleic acids in cells are associated with proteins, which form ionic interactions with the negatively charged phosphates.

Cells and extracellular fluids in organisms contain small concentrations of nucleosides, combinations of a base and a sugar without a phosphate. Nucleotides are nucleosides that have one, two, or three phosphate groups esterified at the 5' hydroxyl. Nucleoside monophosphates have a single esteri-fied phosphate (see Figure 2-14a); diphosphates contain a pyrophosphate group:

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