As scientists learn to read the instructions in our genes, they are discovering that much of our DNA is riddled with errors.

Fortunately, most of these errors are harmless. Considering the difficulties involved—the 6 feet of DNA in a human cell consists of 6 billion subunits, or base pairs, coiled and tightly packed into 23 pairs of chromosomes, all of which must be duplicated every time a cell divides—our general state of health is something of a miracle.

We each inherit hundreds of genetic mutations from our parents, as they did from their forebears. In addition, the DNA in our own cells undergoes an estimated 30 new mutations during our lifetime, either through mistakes during DNA copying or cell division or, more often, because of damage from the environment.

Bits of our DNA may be deleted, inserted, broken, or substituted. Most mutations affect only the parts of DNA that do not contain instructions for making a gene, so we need not worry about them. Problems arise only when an error in DNA alters a message that tells certain cells to manufacture a certain protein. Such messages are spelled out in varying sequences of the four chemical bases that make up DNA: adenine (A), thymine (T), guanine (G), and cytosine (C).

To stay alive and functioning, the human body requires a daily crop of billions of fresh protein molecules—about 40,000 different kinds of proteins that must be supplied in the right quantities, at the right times, and in the right places. We need hemoglobin to carry oxygen through the bloodstream, antibodies to fight foreign substances, hormones to deal with stress, neurotransmitters to evoke movements, emotions, and thought, and many other proteins to give structure to organs or speed up chemical reactions.

Much of the recent progress in reading DNA has come from analyses of genetic errors.

— Maya Pines

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Most human cells divide and multiply repeatedly during a lifetime. Before each cell division, all the DNA in the nucleus must be duplicated.

Illustration: Karen Barnes — Stansbury Ronsaville Wood, Inc.

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