Biorientation

Biorientation is the arrangement of replicated chromosomes on the mitotic spindle in which sister chromatids are attached to microtubules emanating from opposite spindle poles. This configuration places tension across the centromeres, signaling that chromosomes are correctly aligned for segregation.

During early mitosis, microtubules grow and shrink and attach to kinetochores at the centromeres. Proper biorientation, also called amphitelic attachment, occurs when each sister kinetochore binds microtubules from opposite poles. Tension across the sister centromeres stabilizes these attachments and promotes progression to anaphase. Cells employ error-correction mechanisms, notably involving Aurora B kinase, that destabilize incorrect attachments (such as both kinetochores attached to the same pole, or a single kinetochore attached to microtubules from both poles) until correct biorientation is achieved.

The spindle assembly checkpoint monitors the attachment status and tension of kinetochores. If kinetochores remain unattached or lack proper tension, the checkpoint delays anaphase onset to prevent chromosome missegregation.

Accurate biorientation is essential for equal distribution of chromosomes to daughter cells. Errors can lead to aneuploidy and are implicated in developmental abnormalities and cancer. Different types of attachment errors include syntelic, merotelic, and monooriented attachments, each requiring correction before anaphase proceeds.

In summary, biorientation ensures that sister chromatids are segregated to opposite poles by establishing correct kinetochore–microtubule attachments and enabling the spindle assembly checkpoint to safeguard chromosome number in daughter cells.