The process of cell division for somatic cells and for the ovum after fertilisation. Each CHROMOSOME becomes doubled by splitting lengthwise and forming two chromatids which remain held together by the centromere. These chromatids are exact copies of the original chromosomes and contain duplicates of all the genes they bear. When cell division takes place, each double chromatid separates, one passing to one pole of the nucleus and the other to the opposite pole. The nucleus and the cell itself then also divide, forming two new daughter cells containing precisely the same 23 pairs of chromosomes and carrying exactly the same complement of genes as did the mother cell. (See CHROMOSOMES; FERTILISATION; GENES; HEREDITY.)