Because micronuclei originating from the two pathways depicted in Fig. 1 are morphologically identical, the preferential mechanism of action (i.e., clastogenic vs aneuploidogenic activity) of many agents of environmental importance cannot be directly defined. With the advent of molecular cytogenetics, different strategies were used to distinguish between micronuclei originating from chromosome breaks or chromosome loss: the key DNA sequences are the centromeric and telomeric sequences because their presence in the micro-nucleus can demonstrate the preservation of chromosome integrity. Centromere-bearing micronuclei derive from chromosome loss (Fig. 1B) and can be considered for simplicity as whole chromosomes, even if the possibility that they harbor a chromosome with terminal deletion cannot be excluded. However, in this remote situation, the reason for micronucleus formation still would be the segregation error, as demonstrated by the presence of the centromeric sequence. Remember that the alternative modality of micronucleus formation, based on a clastogenic action, implies that the chromosome fragment lacks the centromere (it would be an acentric fragment; Fig. 1A). For a better definition of micronucleus content and, consequently, of the mechanisms of induced chromosome damage in mammalian somatic and germ cells, dual-color approaches for the simultaneous localization of centromeric, pericentromeric, and telomeric sequences can be applied. Figure 2 exemplifies the possible molecular patterns expected in micronuclei after single- and dual-color detection of the aforementioned repetitive DNA sequences.
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