Fig. 1. The Human Genome at Four Levels of Detail. Apart from reproductive cells (gametes) and mature red blood cells, every cell in the human body contains 23 pairs of chromosomes, each a packet of compressed and entwined DNA (1, 2). Each strand ofDNA consists of repeating nucleotide units composed of a phosphate group, a sugar (deoxyhbose), and a base (guanine, cytosine, thymine, or adenine) (3). Ordinahly, DNA takes the form of a highly regular double-stranded helix, the strands of which are linked by hydrogen bonds between guanine and cytosine and between thymine and adenine. Each such linkage is a base pair (bp); some 3 billion bp constitute the human genome. The specificity of these base-pair linkages underlies the mechanism of DNA replication illustrated here. Each strand of the double helix serves as a template for the synthesis of a new strand; the nucleotide sequence (i.e.. linear order of bases) of each strand is sthctly determined. Each new double helix Is a twin, an exact replica, of its parent. (Figure and caption text provided by the LBL Human Genome Center.)
