Two major roles of chiasmata during meiosis I.

(A) Chiasmata eliminate the bipolar attachment of sister centromeres (centromeres on left sister chromatids) during the pre-anaphase stage of meiosis I. (B) When the bipolar attachment remains during anaphase, chiasmata generate bias in the poleward pulling forces to cause proper chromosome segregation. White arrows indicate the pulling forces exerted on chromosomes during anaphase I. A smaller arrow indicates a weaker or less continuously exerted force. For simplicity, only a single microtubule is shown to illustrate the spindle attachment of each kinetochore.