A human has 22 pairs of nonsex chromosomes (autosomes) and two sex chromosomes—two X chromosomes in a female or an X chromosome and a Y chromosome in a male. Each autosome or X or Y chromosome is composed of a long DNA molecule constructed as a double helix (Figure 1-1). Each component strand of the double helix is a chain of nucleotides of four types designated by the names of the bases adenine (A), cytosine (C), guanine (G), and thymine (T). The nucleotides bond, A to T and C to G, between the two strands of the helix like the rungs of a ladder or, better, the steps in a spiral staircase. A pair of complementary nucleotides (or bases)—A-T, G-C, T-A, or C-G—is called a basepair (bp). DNA replication, which takes place in association with cell division, involves the separation of the two strands of the double helix and the synthesis of a new strand of nucleotides complementary to each strand.
Genes are segments of the DNA molecule. They constitute the blueprint for the structure of proteins of various types that are responsible for the makeup and function of cells and the body as a whole. A human has 50,000-100,000 genes, each occurring in every nucleated body cell. Chromosome 1, the largest, might, for example, have about 5,000 genes spaced at intervals along the DNA molecule that it consists of.