To understand what it means to map the entire human genome, it is helpful to start with the biological unit being mapped: our genes. Genes are conceived as segments of a six-foot-long molecule called deoxyribonucleic acid, or DNA, that encodes instructions for making proteins. Each nucleus of the trillions of cells in the human body contains two copies of twenty-three DNA strands, one from each parent, composing forty-six chromosomes. Although each cell has a complete set of DNA, only the portions that are important to the cell’s particular function are activated.
