Holocentric chromosomes

Holocentric chromosomes possess multiple kinetochores along their length rather than the single centromere typical of other chromosomes. They have been described for the first time in cytogenetic experiments dating from 1935 and, since this first observation, the term holocentric chromosome has referred to chromosomes that: i. lack the primary constriction corresponding to centromere observed in monocentric chromosomes; ii. possess multiple kinetochores dispersed along the chromosomal axis so that microtubules bind to chromosomes along their entire length and move broadside to the pole from the metaphase plate. These chromosomes are also termed holokinetic, because, during cell division, chromatids move apart in parallel and do not form the classical V-shaped figures typical of monocentric chromosomes. Holocentric chromosomes 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. As a consequence of their diffuse kinetochores, holocentric chromosomes may stabilize chromosomal fragments favouring karyotype rearrangements. However, holocentric chromosome may also present limitations to crossing over causing a restriction of the number of chiasma in bivalents and may cause a restructuring of meiotic divisions resulting in an inverted meiosis.