The region at which the two sister chromatids are attached after a chromosome has replicated into two identical strands in preparation for cell division. During anaphase of mitosis and of the second meiotic division, the two chromatids detach at the centromere, each moving to opposite sides of the dividing cell. Before this occurs, the pairs of chromatids often look like the letter X; the cetromere is the point at which the two arms of the X cross.