The mitotic spindle: a self-made machine

Abstract

The mitotic spindle is a highly dynamic molecular machine composed of tubulin, motors, and other molecules. It assembles around the chromosomes and distributes the duplicated genome to the daughter cells during mitosis. The biochemical and physical principles that govern the assembly of this machine are still unclear. How-ever, accumulated discoveries indicate that chromosomes play a key role. Apparently, they generate a local cytoplasmic state that supports the nucleation and growth of microtubules. Then soluble and chromosome-associated molecular motors sort them into a bipolar array. The emerging picture is that spindle assembly is governed by a combination of modular principles and that their relative contribution may vary in different cell types and in various organisms. T he mitotic spindle is an aestheticallyattractive structure (Fig. 1A). It is alsoa molecular machine capable of dis-tributing the genome to the daughter cells with stunning precision. The spindle is built of microtubules that are used as tracks to move chromosomes precisely during cell di