Micronucleus

rdf:langString Micronucleus

rdf:langString Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal instability. Micronuclei are commonly seen in cancerous cells and may indicate genomic damage events that can increase the risk of developmental or degenerative diseases. Micronuclei form during anaphase from lagging acentric chromosome or chromatid fragments caused by incorrectly repaired or unrepaired DNA breaks or by nondisjunction of chromosomes. This incorrect segregation of chromosomes may result from hypomethylation of repeat sequences present in pericentromeric DNA, irregularities in kinetochore proteins or their assembly, dysfunctional spindle apparatus, or flawed anaphase checkpoint genes. Micronuclei can contribute to genome instability by promoting a catastrophic mutational event called chromothripsis. Many micronucleus assays have been developed to test for the presence of these structures and determine their frequency in cells exposed to certain chemicals or subjected to stressful conditions. The term micronucleus may also refer to the smaller nucleus in ciliate protozoans, such as the Paramecium. In mitosis it divides by fission, and in conjugation a pair of gamete micronuclei undergo reciprocal fusion to form a zygote nucleus, which gives rise to the macronuclei and micronuclei of the individuals of the next cycle of fission.