Every eukaryotic species has a characteristic number of chromosomes (chromosome number). In species that reproduce asexually, the chromosome number is the same in all the cells of the organism. Among sexually reproducing organisms, the number of chromosomes in the body (somatic) cells is diploid (2n; a pair of each chromosome), twice the haploid (1n) number found in the sex cells, or gametes. The haploid number is produced during meiosis. During fertilization, two gametes combine to produce a zygote, a single cell with a diploid set of chromosomes. See also polyploidy.
Somatic cells reproduce by dividing, a process called mitosis. Between cell divisions the chromosomes exist in an uncoiled state, producing a diffuse mass of genetic material known as chromatin. The uncoiling of chromosomes enables DNA synthesis to begin. During this phase, DNA duplicates itself in preparation for cell division.