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Each DNA strand is made up of smaller pieces, or subunits called bases, which are joined together in a long row like beads on a necklace. In the same way that a word carries information by the order of its leers, DNA carries information by the order of its bases, which form a code. We use 26 different leers to make words, but DNA has only 4 different bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Genes make up sections of the DNA double helix, and each gene has hundreds or thousands of bases in sequence, so it can carry a huge amount of information. The whole set of human genes has 3.1 billion pairs of bases. For a gene to work, the order of its bases is copied into another substance very similar to DNA, called ribonucleic acid (RNA). The RNA then goes to another place inside the cell, where it carries out its job, almost like a person in a factory. It gathers together various substances or raw materials and fixes them together in the right order, following the instructions in the original gene. GENE PRODUCTS

DNA is composed of two long strands that make a double-helix shape, like a twisted ladder. The bases that make up the “le‡ers” for the code of genetic information are like the rungs of the ladder.

The result of this process is a product—a substance or protein in the body. An example is the substance that gives the eyes their color. In one person, the product is blue, so the eyes

are blue. We say that the person has the gene for blue eyes. In another person, the gene for eye color has a slightly different order of bases. This gives a brown product for brown eyes. A whole human body has about 19,000 different types of genes. Every tiny cell contains all of these genes. But in any single cell, only some of the genes are “switched on.” The rest are “switched off.” In the cells at the front of Chromosomes (shown highly magnified here) are arranged and inherited in pairsone from each parent. Humans have 23 pairs of chromosomes in every cell.

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