The 25,000 genes scattered throughout the human chromosomes comprise only about 3 percent of the total genome. These genes hold information critical to all human life. While all the component bases in a gene are copied as information leaves the nucleus, not all this information is kept. This is because within a gene there are both coding and noncoding stretches of bases. For example, in split genes, coding sections called exons supply the genetic instructions that are copied to direct protein building. These sections are preserved, but other noncoding sections within the gene, called introns, are rapidly removed and degraded.

Close to each gene is a "regulatory" sequence of DNA, which is able to turn the gene "on" or "off." Farther away, there are enhancer regions, which can speed up a gene's activity.

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