During meiosis I and II, errors like nondisjunction cause chromosomes to fail to separate, producing aneuploid daughter cells with absent or extra chromosomes. Nondisjunction occurs at anaphase I or II of meiosis. During anaphase I, homologous chromosomes separate and get dragged towards opposite poles. If a chromosome pair fails to detach, both are pulled to one end. Nondisjunction may result from mutations in the proteins of the synaptonemal complex that holds the chromosome together, asymmetric spindle fibers, or misorientation of chromosomes. As a result, four haploid daughter cells are formed, two with an additional chromosome and two lacking a chromosome. Similarly, during anaphase II of meiosis II, sister chromatids may remain affixed and move to the same pole. Such nondisjunction arises from defects in topoisomerase II or condensins that are essential for chromosome separation. These errors yield four haploid daughter cells, two with a standard set of chromosomes and two aneuploids.