The Contractile Ring

The final step of the cell cycle that divides a cell into two daughter cells is called cytokinesis. Cytokinesis begins after chromosome separation in mitosis and ends when the cell divides.

The beginning of cytokinesis is marked by the appearance of a crease, called the cleavage furrow. Starting in anaphase, the furrow deepens and spreads to form a ring around the cell. This compression, which ultimately divides the cell into two, is generated by the contractile ring.

A protein called RhoA is the chief regulator of contractile ring assembly and function. To ensure the contractile ring is formed in the right place, RhoA is activated locally at the cell cortex, near the equator of the cell. RhoA, along with anaphase spindle fibers, also ensures that the contractile ring is formed at the right time, after chromosome segregation.

The contractile ring is made up of structural proteins, including actin filaments and myosin II filaments. RhoA activity results in the assembly of myosin II and anti-parallel actin filaments into the structure of the contractile ring. RhoA promotes localized actin filament polymerization which is necessary for the contractile ring formation.

The contractile ring components generate the force necessary to divide the cell. One mechanism of contraction involves myosin motor activity. Here, myosin filaments move toward the plus end of adjacent antiparallel actin filaments.  This activity pulls the anti-parallel actin, causing them to slide past one another, contracting the ring.

The contraction of the ring continues until it pinches off two new cells. The ring is degraded once the new cells are formed.

Inefficient or absence of contractile ring formation can lead to abnormal cell division, impaired growth, and the potential for tumor formation.