2.20–2.22: Nucleic acids store information on how to build and run a body.

Spirals Time—Time Spirals: a sculpture inspired by DNA’s double helix structure.

We have examined three of life’s macromolecules: carbohydrates, lipids, and proteins. We turn our attention now to the fourth: nucleic acids, macromolecules that store information. There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Both play central roles in directing the production of proteins in living organisms, and by doing so play a central role in determining all of the inherited characteristics of an individual.

Nucleic acids and are made up of individual units called nucleotides. All nucleotides have three components: a molecule of sugar, a phosphate group (containing a phosphorus atom bound to four oxygen atoms), and a nitrogen-containing molecule (FIGURE 2-45). In both types of nucleic acids, nucleotides are linked in a series to form a ribbon-like strand that is the backbone of the nucleic acid molecule: a sugar molecule is attached to a phosphate group, which is attached to another sugar, which is attached to another phosphate, and so on. Attached to each sugar, and protruding from the backbone, is one of the nitrogen-containing molecules called DNA or RNA bases (so named because of their chemical structure). A 10-unit nucleic acid strand therefore would have 10 bases, one attached to each sugar within the sugar-phosphate-sugar-phosphate backbone. But the base attached to each sugar is not always the same. It can be one of several different bases. For this reason, a nucleic acid is often described by its sequence of bases.

Figure 2.45: The molecules that carry genetic information. The nucleic acid shown here is DNA.

Nucleic acids store information in the order of bases attached at each position in the molecule’s backbone. At each position in a molecule of DNA, for example, the base can be any one of four possible bases: adenine (A), thymine (T), guanine (G), or cytosine (C). Just as the meaning of a sentence is determined by which letters are strung together, the information in a segment of DNA is determined by its sequence of bases. One DNA segment may have the sequence adenine, adenine, adenine, guanine, cytosine, thymine, guanine—abbreviated as AAAGCTG. Another DNA segment may have the sequence CGATTACCCGAT. Because the information differs in each case, so, too, does the polypeptide for which the sequence codes, as we’ll see.