Holocentric chromosome

Holocentric chromosomes are chromosomes that possess multiple kinetochores along their length rather than the single centromere typical of other chromosomes.^[1] They were first described in cytogenetic experiments in 1935. Since this first observation, the term holocentric chromosome has referred to chromosomes that: i) lack the primary constriction corresponding to the centromere observed in monocentric chromosomes;^[2] and ii) possess multiple kinetochores dispersed along the entire chromosomal axis, such that microtubules bind to the chromosome along its entire length and move broadside to the pole from the metaphase plate.^[3] Holocentric chromosomes are also termed holokinetic, because, during cell division, the sister chromatids move apart in parallel and do not form the classical V-shaped figures typical of monocentric chromosomes.^[4][5][6]

Holocentric chromosomes have evolved several times during both animal and plant evolution, and are currently reported in about eight hundred diverse species, including plants, insects, arachnids, and nematodes.^[7][8] As a consequence of their diffuse kinetochores, holocentric chromosomes may stabilize chromosomal fragments created by accidental double-strand breaks, preventing loss of the fragments and favouring karyotype rearrangements.^[9][10] However, holocentric chromosomes may also present limitations to crossing over, causing a restriction of the number of chiasma in bivalents,^[11] and may cause a restructuring of meiotic divisions resulting in an "inverted" meiosis.^[12]