After the duplicated chromosomes have condensed, a pair of complex cytoskeletal machines assemble in sequence to carry out the two mechanical processes that occur in M phase.

The mitotic spindle carries out nuclear division (mitosis), and, in animal cells and many unicellular eukaryotes, the contractile ring carries out cytoplasmic division (cytokinesis). Both structures disassemble rapidly after they have performed their tasks.

The mitotic spindle is composed of microtubules and the various proteins that interact with them, including microtubule-associated motor proteins. In all eukaryotic cells, the mitotic spindle is responsible for separating the duplicated chromosomes and allocating one copy of each chromosome to each daughter cell.

The contractile ring consists mainly of actin and myosin filaments arranged in a ring around the equator of the cell. It starts to assemble just beneath the plasma membrane toward the end of mitosis. As the ring contracts, it pulls the membrane inward, thereby dividing the cell in two. We discuss later how plant cells, which have a cell wall to contend with, divide their cytoplasm by a very different mechanism.