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.