Mitosis
Mitosis in a diploid organism
    The diagram shows the process of mitosis in a diploid cell with 2n = 2. There are two chromatids in two chromosomes, each with a centromere. The cell is heterozygous at the A locus (Aa). In the second stage, DNA replication accompanies chromosome duplication: there are now four chromatids and two chromosomes (as defined by the number of centromeres). In the third stage, the two chromosomes line up on the cell equator. In the fourth stage, the two chromatids of each chromosome are pulled apart by microfilaments attached to the centromere, so that one goes to either cell pole. In the fifth stage, the two daughter cells separate, each with the same pair of alleles as the parent cell. Note that semi-conservative replication of the DNA means that one of the chromatids in one daughter cell is new and the other one is old.
Figure ©2002 by Griffiths et al.; all text material ©2014 by Steven M. Carr