E Summary of biparental meiosis. In normal-sized oocytes, biparental meiosis frequently exhibits premature separation of paternal chromosomes into univalent-like structures. These chromosomes undergo predivision (premature segregation of sister chromatids), and thus result in separated chromatids in MII oocytes. Chromosome nondisjunction and breakage were relatively minor. Halving the ooplasmic volume reduced premature separation of paternal chromosomes. 