Our cells are kept extremely busy linking together amino acids—the building blocks of proteins—in the right order to produce varying quantities of the 40,000 different proteins we need every day.

The order of these amino acids is determined by the genes. According to the genetic code, which was deciphered in the 1960s, each triplet of bases in the genes' instructions either calls for a particular amino acid or gives a signal to start or stop making a protein.

An error in just one base can bring the wrong amino acid, altering the protein. And should one or two bases be missing, each succeeding triplet will be read in the wrong combination; such "reading-frame shifts" generally prevent cells from making the protein at all.

Actually the DNA's instructions are not transmitted directly; a copy made of ribonucleic acid (RNA) acts as an intermediary. The original DNA remains safely in the nucleus, somewhat like the printing block in a printing press, while the RNA copy is produced by transcribing just one strand of DNA, which carries the genetic instructions.

Reading the DNA of humans and other mammals is complicated by the astonishing fact—discovered over a decade ago—that the instructions for making a protein are split into separate segments of DNA. These instructions must be spliced together before they can be carried out by a cell. Only about 5 percent of the DNA in mammalian genes actually contains the recipe for making a protein. The remaining 95 percent consists of intervening sequences, or "introns," whose function is unknown.

Splicing together the "exons"—the protein-coding sequences—is a very delicate, precise operation that involves snipping out the introns to end up with a much shorter strand of potent RNA. At the exon–intron boundaries are splicing signals, which researchers can now identify. Several genetic diseases have been traced to disrupted splicing.

— Maya Pines

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