The centromere is a highly specialized domain on a chromosome that controls the process of faithfull sister chromatid segregation during mitosis and meiosis. The common feature of the centromeric DNA is the presence of long tandem arrays in which the monomers are species-specific. Centromere identity is epigenetically determined by the presence of centromere protein CENP-A and its homologues, conserved histone H3 variants. Neocentromeres are the structures devoid of complete activity characteristic of centromeres. Neocentromeres originate from ectopic, noncentromeric region of a chromosome. It is possible that neocentromere formation depends on the inactivation of an endogenous centromere. Inactivation of endogenous centromere consists in loss of centromeric constitutive proteins, with exception of CENP-B (if present). In dicentric chromosomes formed by fusion of two chromosome arms belonging to the same chromosome, each bearing one centromere, therefore containing the same kind of centromeric DNA, the switching of the centromere activity seems to be due to epigenetic changes, e.g. as a result of histone hyperacetylation. Neocentromeres are formed mainly on acentric chromosome fragments, therefore such a chromosome behaves during mitosis and meiosis as a normal one. Despite the absence of centromeric DNA, neocentromeres are able to assemble all the centromere and kinetochore proteins. Human neocentromeres are formed mainly as a result of chromosome rearrangements, on chromosomal acentric arm fragments, but neocentromeres can also originate on unarranged chromosomes. Till 2004, 70 neocentromeres have been identified on human chromosomes. Non-random distribution of neocentromeres in the human genome has been observed. Disproportional number of neocentromeres is formed on 3q, 13q and 15q. Formation of centromere on 4q21 is not preceded by chromosome rearrangement. Formation of neocentromeres can be induced in vitro on mammalian minichromosomes and human artificial chromosomes (HACs). Upon introduction by transfection or microinjection of centromere DNA repeats into several kinds of cells, functional neocentromeres are formed. Neocentromeres on human minichromosomes and on HACs are formed after transfection of human centromeric alphoid DNA repeats containing CENP-B boxes elements into cells in tissue culture. In plants, true neocentromeres, i.e. devoid of CENH3 protein (homologue of CENP-A) and other centromere and kinetochore proteins, similar to those described in human chromosomes. They are formed in response to chromosome rearrangements. The term of "neocentromere" is used also to denote the structures being large heterochromatic domains - knobs - that contain two kinds of DNA tandem repeats, 350 bp and 180 bp. Knobs associate with microtubules and move rapidly poleward during 2(nd) anaphase in meiosis and function together with an endogenous, true centromere. These structures contain neither CENH3, nor other centromeric proteins and are described in detail only in abnormal chromosome 10 in maize. In endopoliploid nuclei, unable to enter mitosis, in spite of proportional centromere DNA multiplication, the amount of CENH3 does not increase proportionally to the level of centromeric DNA. Human neocentromeres that originate on euchromatic bands of a chromosome, contain some unique sequences of DNA, similary as endogenous centromere of rice chromosome 8. Contrary to canonical endogenous centromeres,. transcriptional competence has been demonstrated of both types of centromere, i.e. human neocentromeres and centromere of rice chroosome 8.
