A quantitative study of chromosomal elasticity and its influence on chromosome movement

Summary

Chromosome elasticity and movement have been studied in living cells in two distinct situations: early anaphase stretch due to opposed external forces, and drag stretch — an elongation due to frictional resistance or drag on a chromosome being pulled toward one pole. Drag stretch provides a simultaneous display of both friction and elasticity and shows that chromosomes in living cells are elastic up to approximately six-fold increases in length.

Neither early anaphase stretch nor drag stretch produce detectable alterations in the velocity of chromosome movement. A simple mechanical model is described which permits interpretation of this result for drag stretch: no matter how extensive, drag stretch should produce no change in the force required to maintain a given velocity of movement and hence should not alter movement velocity. Early anaphase stretch is a very different proposition, and additional assumptions leading to a quantitative model are necessary for its interpretation. Nevertheless it is reasonably certain that the amount of stretch actually seen in these circumstances would influence chromosome movement if the applied force were not increased over that necessary in the absence of stretch. It is concluded that the mitotic forces are continually adjusted to produce a standard velocity of movement even when an unusual hindrance to movement exists. The implications of this are considered, particularly in regard to the stretching and rupture of dikinetochoric (“dicentric”) bridges in anaphase.

The quantitative version of the mechanical model for elasticity and movement can be applied to the drag stretch data, and permits calculation of the ratio between frictional and elastic coefficients. The chief assumptions are that the elasticity is Hookian, and the frictional resistance Newtonian in character. The model has not been critically tested, but it is consonant with existing data.