Movement of chromosomes with severed kinetochore microtubules

Abstract

Experiments dating from 1966 and thereafter showed that anaphase chromosomes continued to move poleward after their kinetochore microtubules were severed by ultraviolet microbeam irradiation. These observations were initially met with scepticism as they contradicted the prevailing view that kinetochore fibre microtubules pulled chromosomes to the pole. However, recent experiments using visible light laser microbeam irradiations have corroborated these earlier experiments as anaphase chromosomes again were shown to move poleward after their kinetochore microtubules were severed. Thus, multiple independent studies using different techniques have shown that chromosomes can indeed move poleward without direct microtubule connections to the pole, with only a kinetochore 'stub' of microtubules. An issue not yet settled is: what propels the disconnected chromosome? There are two not necessarily mutually exclusive proposals in the literature: (1) chromosome movement is propelled by the kinetochore stub interacting with non-kinetochore microtubules and (2) chromosome movement is propelled by a spindle matrix acting on the stub. In this review, we summarise the data indicating that chromosomes can move with severed kinetochore microtubules and we discuss proposed mechanisms for chromosome movement with severed kinetochore microtubules.

Elastic tethers between separating anaphase chromosomes in crane‐fly spermatocytes coordinate chromosome movements to the two poles

“…Cutting kinetochore fibres of anaphase chromosomes with the laser microbeam did not cause changes in the velocity of chromosome movements toward their poles, similar to when the kinetochore microtubules were cut by UV microbeam irradiations [Spurck et al, ]. The laser cuts kinetochore microtubules in these cells, e.g., Forer et al [], as it does in other cells (reviewed in Forer et al []), and to test whether the presence of tethers prevents chromosomes from accelerating when their kinetochore microtubules are cut, we cut the tethers before irradiating the kinetochore fibres. We usually irradiated the kinetochore fibres in two separate positions along the length of the fibre.…”

“…We assume that the temporary increased velocity of the chromosomes was because of cutting the kinetochore microtubules, since this is what occurs in other cells (review in Forer et al []). To confirm this, we stained irradiated cells with anti‐tubulin antibody.…”

“…Our experiments show that when kinetochore microtubules in anaphase crane‐fly spermatocytes are cut in the absence of tethers between partner chromosomes, the associated chromosome temporarily speeds up. In previous experiments (described in Forer et al []), when kinetochore microtubules are cut in anaphase crane‐fly spermatocytes in the presence of tethers between partner chromosomes, the associated chromosome moves at the original speed. Our results suggest therefore that tethers coordinate the movements of separating partner chromosomes: severing the tethers unlinks their movements and allows the acceleration.…”

Calibration of optical tweezers for force microscopy

“…Although the microtubules are attached to the poles and the chromatids, it was generally accepted in literature that it was these microtubules which pulled the chromosomes apart; however, chromosomes with severed microtubules still moved. This leaves the mechanisms by which chromosomes move during cell division as a mystery [Forer et al, 2015].…”

“…As discussed in Forer et al [2015], there are two proposed mechanisms that are responsible for the movement of chromosomes: the kinetochore stub interacts with non-kinetochore microtubules, or a spindle matrix acts on the stub. Earlier studies [Nicklas, 1963[Nicklas, , 1965 had shown that there is independence of the mitotic forces acting on the chromosomes and the load required.…”