Before meiosis, the 46 chromosomes are duplicated such that each consists of two sister chromatids. During meiosis I, the homologous chromosomes then partner up into 23 pairs. A single pair of homologous chromosomes is shown (top), and the two chromosomes are connected via crossovers called “chiasmata”. The kinetochores (shown as black semi-circles) on sister chromatids are fused together and become attached to microtubules (shown in green) emanating from the same pole of the spindle (not shown). This geometry helps the kinetochores of pairs of homologous chromosomes to be captured by microtubules from opposite spindle poles, and pulled to opposite sides of the cell. Zielinska et al. occasionally observed abnormal configurations of chromosomes in human oocytes. Often the sister kinetochores were split and ended up attached to spindle microtubules from different spindle poles (bottom left). This allowed the pair of homologous chromosomes (or "bivalent”) to rotate. Also, the connections between homologous chromosomes were commonly compromised, which sometimes led to premature separation of the bivalent (bottom right).