Cytokinesis is the process of cell division through which the cytoplasm of a single eukaryotic cell separates into two daughter cells. It begins towards the end of mitosis and involves three main steps: 1) formation of the contractile ring composed of actin and myosin filaments underneath the cell membrane; 2) constriction of the contractile ring which furrows the cell membrane; 3) completion of cleavage with separation of the two daughter cells connected by a midbody.

1. Cytokinesis
Somanna A. N.

2. Scanning electron micrographs of early cleavage in a fertilized frog egg. The
furrowing of the cell membrane is caused by the activity of the contractile ring
underneath it.

3. The contractile ring

4. A schematic drawing of a cleavage furrow in a dividing cell.

5. (A) Scanning electron micrograph of an animal cell in culture in the
process of dividing; the midbody still joins the two daughter cells. (B) Electron micrograph
of the midbody of a dividing animal cell. Cleavage is virtually complete, but the daughter
cells remain attached by this thin strand of cytoplasm.
The midbody reveals a dense matrix of tightly packed polar microtubules remaining from
the mitotic spindle.

6. Activation of RhoA triggers assembly and contraction of
the Contractile Ring

7. (a) Microtubule disassembly stimulates RhoA activity (left). Rho stabilizes microtubules through the
formin mDia and also results in actin-myosin contraction through stimulation of Rho kinase
(b) Actin (red) and microtubules (green) can exhibit static or dynamic interactions. Interaction 1
shows a protein that possesses both actin- and microtubule-binding sites and could provide a static
crosslink between the two polymers. Interaction 2 shows a complex between an actin-based motor
(blue) and a microtubule-based motor (orange), whereas interaction 3 shows a complex between a
motor (yellow) and a binding protein (pink). Both types of interaction could move actin and
microtubules relative to one another.

8. An experiment that shows the influence of the position of microtubule asters on the subsequent plane of cleavage. If a
mitotic spindle is mechanically pushed to one side of the cell, the membrane furrowing is incomplete, failing to occur on the
opposite side of the cell. Subsequent cleavages occur not only in the conventional relation to each of the two subsequent
mitotic spindles (yellow arrowheads) but also between the two adjacent asters that are not linked by a mitotic spindle (but
in this abnormal cell share the same cytoplasm)(red arrowhead). Apparently, the contractile bundle of actin filaments that
produces the cleavage furrow always forms in the region midway between two asters, which implies that the asters
somehow alter the adjacent region of cell cortex.

9. The phases of cytokinesis or cleavage