In eukarya, as in bacteria and archaea, single cells reproduce asexually via a cell cycle. The cell cycle of eukarya, though, is more complex and elaborately regulated. The large, linear chromosomes of eukarya, whose DNA is packaged in chromatin, are replicated once per cycle, and copies are apportioned to daughter cells with great accuracy in a nuclear division phase, mitosis. The phases of the cell cycle and the main points of cell-cycle regulation are described in this chapter.
Keeping Track of Chromosome Number
Ploidy refers to the number of chromosome sets per cell nucleus. A euploid cell has an integer multiple of chromosome sets. Normal sex cells are haploid, having one set of chromosomes; sex cells unite to make zygotes, diploid cells with two sets of chromosomes.
In sexually reproducing eukarya, the haploid chromosome number of a species is N, and the diploid number is 2N. A gamete has N chromosomes, and most somatic cells of plants and animals have 2N chromosomes. Some cells, though, are polyploid – i.e., they have three or more sets of chromosomes. For example, in humans N = 23; a sperm has 23 chromosomes, a skin cell has 46 chromosomes, and a liver cell has 92 or more chromosomes. Mitosis works the same no matter what the cell's ploidy.
However – and this is important – the cell's ploidy normally does not change with the phase of the cell cycle, even though the number of copies of the genome does change.