The first step in cytokinesis is the determination of where the contractile ring will form. The contractile ring is a structure that functions similarly to a muscle. The filaments that make up the contractile ring squeeze tighter and tighter, pinching the cell until it is divided in two. Positioning of the contractile ring is determined according to the positioning of the spindle. Spindles are the cellular structures responsible for separating chromosomes. Contractile rings are formed perpendicular to spindles at their midpoint.
The second step of cytokinesis is the actual assembly of the filaments that make up the contractile ring. These filaments are composed of a protein called actin. Actin is plentiful within cells and is continually broken down and rebuilt as needed. Actin filaments used during this step are most likely recruited during earlier stages of mitosis.
During the third step of cytokinesis the contractile ring contracts to divide the cell in half. A protein called myosin causes the actin filaments of the ring to move past one another to cause contraction. This process is very similar to the process by which muscles of the body contract. Some of the actin filaments composing the ring are removed during this step to allow for a decrease in size of the ring.
The final step of cytokinesis is also the final step of cell division. The plasma membrane and cytoplasm are broken down and rebuilt to form new cells. This step solidifies the separation and allows the newly formed daughter cells to function independently.