Fig. 2. Mechanisms of micronuclei and chromosome bridges formation and rupture.

Errors in cell division, which involve physical separation of a chromosome or chromosome region from the rest of the chromosomes (such as a lagging chromosome or acentric chromosome fragments), lead to the formation of a micronucleus in the subsequent interphase. Dicentric chromosomes result in the formation of a chromosome bridge when two centromeres are pulled apart to the opposite poles in mitosis; subsequently, chromosome fragments generated following the breakage of the chromosome bridge become micronucleated. Nuclear envelope assembly in micronuclei and chromosome bridges is delayed due to aberrant chromosome separation in anaphase and spindle inhibition. Furthermore, nuclear envelopes of both micronuclei and chromosome bridges are prone to rupture due to reduced levels of some nuclear envelope proteins (including lamin B1), suboptimal numbers of nuclear pores, mechanical tension, and stress due to small radius of curvature, endoplasmic reticulum (ER) invasion and insufficient repair of the rupture sites by the ESCRTIII complex.