Mitotic motors

T HE complex but well-controlled motions of chromosomes during mitosis have long evoked the view that the mitotic spindle contains enzymes capable oftransducing chemical energy into mechanical work, but the nature ofthese "motor" molecules has been elusive . During the last 40 years, three ideas have dominated the field : (a) microtubules (MT)' attach to chromosomes and move them by the addition and removal of tubulin ; (b) mitotic forces are produced by mechanochemical enzymes, probably ATPases, that interact with MTs; and (c) mitotic forces are generated by muscle proteins . Possibility c now seems remote, but recent data from studies on the dynamics of spindle MTs, on the genes and gene products that are required for normal chromosome motion, and on the time-dependent changes in spindle structure reveal that both possibilities a and b are pertinent for explaining mitotic movements. While it is too soon to say just how chromosomes are moved during cell division, there is now enough information to identify key phenomena and the kinds of proteins involved . This paper is a sketch of the motile events that occur during mitosis and a discussion of the ways that MTs and their associated motor enzymes may cause these motions .