Apart from reproductive gametes, each human cell contains 23 pairs of chromosomes, each a packet of DNA. Every strand of DNA is a natural polymer of repeating nucleotide units, each of which comprises a phosphate group, a sugar (deoxyribose), and a base (either adenine, thymine, cytosine, or guanine). Every strand thus embodies a code of four characters (A's, T's, C's, and G's), the recipe for the machinery of human life. In DNA double helix, the strands are linked by hydrogen bonds between adenine and thymine (A,T) and between cytosine and guanine (C, G). Each such linkage is said to constitute a base pair; some three billion base pairs constitute the human genome. Credit: U.S. Department of Energy Genome Programs