Micronuclei are small, extranuclear bodies that are formed during mitosis from lagging chromosomes.

The lagging chromosome is not incorporated into the nucleus of one of the daughter cells, resulting in one normal daughter cell and one with monosomy.

If it is not corrected, the chromatid bound to both poles will remain as a lagging chromosome at anaphase, and finally will be broken in two fragments, distributed between the daughter cells, generating aneuploidy.

Occasionally, one of the two sister kinetochores may attach simultaneously to MTs generated by both poles, a configuration named merotelic, which is not detected by the spindle checkpoint but that may generate lagging chromosomes during anaphase and, consequently, aneuploidy.