consist of two rings; and two pyrimidines, cytosine and thymine, which consist of a single ring (figure 2.23). Each of these bases is linked to the same carbon atom (C1) of deoxyribose as occurs in sugar-sugar bonds.

These four bases are named after the original source from which the base was isolated. Adenine was isolated from the pancreas and is derived from the Greek word for gland. Guanine was isolated from bird guano (excrement). Thymine was isolated from the thymus gland, and cytosine was isolated from cells. Cyt is derived from the Greek word for cell.

The nucleotide subunits are joined by a covalent bond between the phosphate of one nucleotide and the sugar of the adjacent nucleotide (figure 2.24a). Thus, the phosphate is a bridge that joins the number 3 carbon atom (termed 3') of one sugar to the number 5 carbon atom (termed 5') of the other. The result is a molecule with a backbone of alternating sugar and phosphate molecules. The two ends of the molecule are different. The 5' end has a phosphate molecule attached to the sugar; the 3' end has a hydroxyl group (figure 2.24b). Accordingly, the end of the chain that grows by adding more nucleotides is always the 3' end. The synthesis of DNA is covered in chapter 7.

The DNA of a typical bacterium is a single molecule composed of nucleotides joined together and arranged in a double-stranded helix, with about 4 million nucleotides in each strand (figure 2.25). This double-stranded helical molecule can be pictured as a spiral staircase with two railings and stairs split in half. The railings represent the sugar-phosphate backbone of the molecule, and the stairs attached to the railings are the bases. One half of each stair is strongly attached to one railing, and the other half is strongly attached to the other railing. Each pair of stairs (bases) is held together by weak hydrogen bonds. A specificity exists in the bonding between bases, however, in that adenine (A) can only hydrogen bond to thymine (T), and guanine (G) to cytosine (C). The pair of bases that bond are complementary to each other. Thus, G is complementary to C, and A to T. As a result, one entire strand of DNA is complementary to the other strand. This explains why in all DNA molecules, the total number of adenine molecules is equal to the number



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