5.3: Genes are sections of DNA that contain instructions for making proteins.

One of DNA’s most amazing features is that it embodies the instructions for building the cells and structures for almost every single living organism on earth (FIGURE 5-5). Thus, DNA is like a universal language, the letters of which are the bases A, T, C, and G. (Note that the sugar-phosphate backbone serves only to hold the bases in sequence, like the binding of a book. It does not convey genetic information.)

We’ve seen how the structure of DNA is like a spiral staircase. Another analogy may help you understand the information-containing aspect of DNA. You can think of an organism’s DNA as a cookbook. Just as a cookbook contains detailed instructions on how to make a variety of foods (such as french toast, macaroni and cheese, or chocolate chip cookies), an organism’s DNA carries the detailed instructions to build an organism and keep it running. And just as a book can be viewed as a sequence of letters, with the book’s meaning determined by which letters are strung together and in what order, a molecule of DNA can be viewed as a sequence of bases. Letters don’t have much meaning on their own, of course, but when they are put together into words and sentences, their order holds a great deal of information. Similarly, the sequence in which bases appear in a molecule of an organism’s DNA makes up a code that holds the detailed instructions for the building of the organism—chiefly in the form of instructions for the amino acid sequences of polypeptides. Upon processing and folding, these polypeptides become the numerous and varied protein molecules that make up an organism—whether it is a one-celled amoeba, a giant oak tree, or a biology student.

The full set of DNA present in an individual organism is called its genome (FIGURE 5-6). In prokaryotes, including all bacteria, the information is contained within circular pieces of DNA. In eukaryotes, including humans, this information is laid out in long linear strands of DNA in the nucleus. Rather than being one super-long DNA strand, eukaryotic DNA exists as many smaller, more manageable pieces, called chromosomes. Humans, for example, have three billion base pairs, divided into 23 unique pieces of DNA. Because we have two copies of each (one from our mother, one from our father), we have 46 chromosomes.

Within the long sequences of bases in a cell’s DNA molecules are relatively short sequences, on average about 3,000 bases long, that are the actual genes. The location or position of a gene on a chromosome is called a locus (pl. loci). “Gene” may seem like an impossibly nebulous concept, because the word is often used casually in the media as if it were some magical, irresistible, and mysterious force that controls our bodies and behavior. Beyond these vague descriptions, though, the word has a literal meaning. A gene is a sequence of bases (or, more precisely, base pairs) in a DNA molecule that carries the information necessary for producing a functional product, usually a polypeptide or RNA molecule. Nothing more, nothing less.

Remember that a DNA molecule is like a ladder in which half of each rung is any one of the four bases—A, T, G, or C. Strung together, a segment might be read as “AAAGGCTAGGC…” continuing on for another 3,000 or so bases. Returning to our cookbook analogy, just as a particular sequence of letters in a cookbook may be read and understood as the directions for baking chocolate chip cookies, the sequence of bases in DNA also carries information. Perhaps the sequence spells out part of the instructions for producing a red blood cell, or for constructing the keratin that will form part of a curly strand of hair, or for assembling a chemical that alters your brain chemistry so that you exhibit a mood disorder or suicidal behavior.

Each gene is the instruction set for producing one particular molecule, usually a protein. For example, there is a gene in silk moths that codes for fibroin, the chief component of silk. And, there is a gene in humans that codes for triglyceride lipase, an enzyme that breaks down dietary fat.

Within a species, individuals sometimes have slightly different instruction sets for a given protein, and these instructions can result in a different version of the same characteristic. These alternative versions of a gene that code for the same feature are called alleles (FIGURE 5-7)—and function like alternative recipes for chocolate chip cookies. Any single characteristic or feature of an organism is referred to as a trait. A simple hypothetical example will clarify the meaning of these terms. The color of a daisy’s petals is a trait. The instructions for producing this trait are found in a gene that controls petal color. This gene may have many different alleles; one allele may specify the trait of red petals, another may specify white petals, and yet another may specify yellow petals (see Chapter 7). Similarly, one allele for eye color in fruit flies may carry the instructions for producing a red eye, while another, slightly different allele may have instructions for brown eyes. (Ultimately, though, the trait may be influenced not just by the genes an individual carries but by the way those genes interact with the environment, too.)