DNA is physically carried on chromosomes

Humans have 46 chromosomes: two sets of 23 distinct chromosomes. The condition of having two sets of chromosomes (and therefore two sets of the genes carried on them) is called diploidy.

Diploidy is important in reproduction. An adult individual has two sets of chromosomes. Its gametes (eggs in the female, sperm or pollen in the male) have only one set: a human egg, for example, has only 23 chromosomes before it is fertilized.

Gametes are said to be haploid. They are formed by a special kind of cell division, called meiosis.

In meiosis, the two sets of chromosomes are doubled and divided, resulting in gametes with only one set. When male and female gametes fuse, at fertilization, the resulting zygote (the first cell of the new organism) has the double chromosome set restored, and it develops to produce a diploid adult.

This cycle of genesis can then repeat itself. (In some species, organisms are permanently haploid; but we shall mainly be concerned with diploid species. Most familiar, non-microscopic species are diploid.)

The image opposite is of a genome arranged in strings of chromosomes.