Fig. 1. The Human Genome at Four Levels of Detail. Apart from reproductive cells (gametes) and mature red blood cells, every ceil tn the human body contains 23 pairs of chromosomes, each a packet of compressed and entwined DMA (1. 2} Each strand of DMA consists of repeating
nucleotide units composed of a phosphate group, a sugar (deotyrfbosel. and a base (guanine, cytoslne. thymine, or adenine) (3). Ordinarily. DNA takes the form of a highly regular double-stranded hell», the strands of which are finked by hydrogen bonds bettveen guanine and cytoslne and between thymine and adenine. Each such linkage is a base pair (bp); some 3 billion be constitute the human genome The specificity of these base-pair linkages underlies the mechanism ofDNA replication Illustrated here Each strand of the double helix serves as a template for fie synthesis of a new strand: fie nucleotide sequence (I.e.. linear order of bases) of each strand Is strictly 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.)
