FORCES ON CHROMOSOMAL DNA DURING ANAPHASE

In the course of anaphase, the chromosomal DNA is submitted to the tra ction of the spindle. Several physical problems are associated with th is action. In particular, the sister chromatids are generally topologi cally intertwined at the onset of anaphase, and the removal of the int ertwinings results from a coupling between the enzymatic action of typ e II DNA topoisomerases and the force exerted by the spindle. We propo se a physical analysis of some of these problems: 1) We compare the ma ximum force the spindle can produce with the force required to break a DNA molecule, and define the conditions compatible with biological sa fety during anaphase. 2) We show that the behavior of the sister chrom atids in the absence of type II DNA topoisomerases can be described by two distinct models: a chain pullout model accounts for the experimen tal observations made in the budding yeast, and a model of the mechani cal rupture of rubbers accounts for the nondisjunction in standard cas es. 3) Using the fluctuation-dissipation theorem, we introduce an effe ctive protein friction associated with the strand-passing activity of type II DNA topoisomerases. We show that this friction can be used to describe the situation in which one chromosome passes entirely through another one. Possible experiments that could test these theoretical a nalyses are discussed.