Fig. 1. The human genome at four levels of detail. Apart from reproductive 

 cells (gametes) and mature red blood cells, each cell of tfie human body 

 contains 23 pairs of chromosomes, each a packet of compressed and entwined 

 DNA. Each strand of the DNA is a huge natural polymer consisting of repeating 

 nucleotide units — each of which is comprised of a phosphate group, a sugar 

 (deoxyribose), and a base (either guanine, cytosine, thymine, or adenine). In its 

 "normal" state, 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 said to constitute a 

 "base pair": 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 along the DNA strand) of 

 each strand is strictly determined. Each daughter double helix is thus not only a 

 twin, but also an exact replica of its sole parent. (Figure and caption text 

 provided by the Human Genome Center, Lawrence Berkeley Laboratory.) 



120 



