Micronucleus formation during chromatin condensation and under apoptotic conditions

In early S phase the newly replicated DNA is folded back to increasingly compact structures. The process of chromatin condensation inside the nucleus starts with the formation of a micronucleus observed in five established cell lines (K562, CHO, Indian muntjac, murine preB and SCC). Supercoiling of chromatin generates a polarized end-plate region extruded from the nucleus. The extruded chromatin is turned around itself forming the head portion (micronucleus) visible by fluorescence microscopy until the middle of S phase when chromatin structures are succeeded by distinguishable early forms of chromosomes. The generation of micronuclei upon apoptotic treatment was achieved by the methotrexate (MTX) treatment of cells. A close correlation was found between the frequency of micronucleus and MTX concentration, with low frequency at low (0.1 µM) and increasingly higher frequency between 1 and 100 µM concentrations. Characteristic deformation and shrinkage of nuclei indicated apoptosis. High MTX concentration (100 µM) caused the enlargement and necrotic disruption of nuclei. Inhibition of DNA synthesis during replicative DNA synthesis by biotinylated nucleotide prevented the formation of metaphase chromosomes and elevated the frequency of early intermediates of chromosome condensation including micronucleus formation. Based on these observations the micronucleus is regarded as: (a) a regularly occuring element of early chromatin condensation and (b) a typical sign of nuclear membrane damage under toxic conditions. Explanation is given why the micronucleus is hidden in nuclei under normal chromatin condensation and why chromatin motifs including micronuclei become visible upon cellular damage.