chromosome, centromeric region obsolete centromere true OBSOLETE. The region of a eukaryotic chromosome that is attached to the spindle during nuclear division. It is defined genetically as the region of the chromosome that always segregates at the first division of meiosis; the region of the chromosome in which no crossing over occurs. At the start of M phase, each chromosome consists of two sister chromatids with a constriction at a point which forms the centromere. During late prophase two kinetochores assemble on each centromere, one kinetochore on each sister chromatid. GO:0005698 cellular_component This term was made obsolete because it is a genetically defined region and not a specific subcellular localization. centromere