Dna Nucleotides

DNA is a nucleic acid made of two long chains (also called strands) of repeating subunits called nucleotides (NOO-klee-oh-TIEDZ). Each nucleotide consists of three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base. The three parts of a DNA nucleotide are illustrated in Figure 10-6. The five-carbon sugar in a DNA nucleotide is called deoxyribose (dee-AHKS-ee-RIE-bohs). The phosphate group consists of a phosphorus (P) atom bonded to four oxygen (O) atoms. The nitrogenous (nie-TRAHJ-uh-nuhs) base contains nitrogen (N) atoms and carbon (C) atoms and is a base (accepts hydrogen ions).

Bonds Hold DNA Together

The DNA double helix is similar to a spiral staircase, as Figure 10-6 shows. The alternating sugar and phosphate molecules form the side "handrails" of the staircase. Nucleotides along each strand are connected by covalent bonds between the sugar of one nucleotide and the phosphate group of the next nucleotide. Each full turn of the DNA helix has 10 nucleotide pairs.

The nitrogenous bases (called "bases" for short) face toward the center of the DNA molecule. The bases on one strand of DNA face—and form bonds called hydrogen bonds with—the bases on the other strand. Nitrogenous bases are bonded in pairs between the two strands by two or three hydrogen bonds. The base pairs form the "steps" of the staircase. The base pairs are of uniform width because, in each pair one base has a two-ring structure and the other base has a single-ring structure.

In Figure 10-6, dashed lines indicate the locations of the hydrogen bonds. Hydrogen bonds between the bases help hold the two chains of the DNA double helix together.

figure 10-5

Rosalind Franklin and her X-ray diffraction photo of DNA helped reveal the characteristic shape of a double helix.

figure 10-5

Rosalind Franklin and her X-ray diffraction photo of DNA helped reveal the characteristic shape of a double helix.

Hydrogen bonds

Covalent^ bonds

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