The centromere is a special region of a chromosome, usually near the middle. It is where the two identical sister chromatids stay in contact as the chromosome attaches to the spindle in mitosis. The region contains specific types of DNA, which are tandem repetitive sequences (satellite DNA). These sequences bind specific proteins called "cen"-proteins.
During mitosis the centromeres can be seen during the metaphase stage as a constriction at the chromosome. At this centromeric constriction the two halves of the chromosome, the sister chromatids, are held together until late metaphase.