Cytokinesis: Splitting the Cell

Cytokinesis is the physical division of the cytoplasm into two daughter cells. While mitosis divides the nucleus, cytokinesis completes cell division by partitioning the cytoplasm, organelles, and cell membrane. It typically begins in late anaphase or telophase and completes shortly after mitosis ends. The mechanism of cytokinesis differs fundamentally between animal and plant cells due to the presence of a rigid cell wall in plants.

Cytokinesis in Animal Cells

In animal cells, cytokinesis occurs through a process called cleavage. A contractile ring composed of actin filaments and myosin II motor proteins assembles just beneath the plasma membrane at the former position of the metaphase plate. As the ring contracts, it pulls the membrane inward, forming an indentation called the cleavage furrow. The furrow progressively deepens, constricting the cell until only a narrow cytoplasmic bridge — the intercellular bridge — connects the two daughter cells. The final severing of this bridge, known as abscission, is mediated by the ESCRT-III (endosomal sorting complexes required for transport) membrane-remodeling machinery.

Cytokinesis in Plant Cells

Plant cells cannot divide by cleavage because their rigid cell wall — composed primarily of cellulose microfibrils embedded in a matrix of hemicellulose and pectin — prevents the inward pinching required for furrow formation. Instead, plant cells build a new partition from the inside out.

Vesicles derived from the Golgi apparatus, loaded with cell wall polysaccharides and membrane material, are transported along microtubules to the center of the dividing cell. There they fuse to form the cell plate, a disc-like structure that expands outward toward the existing cell wall. As more vesicles fuse, the cell plate matures into a new cell wall complete with a middle lamella, effectively dividing the parent cell into two daughter cells, each enclosed by its own cell wall and plasma membrane.

Molecular Machinery

Contractile Ring Assembly

The assembly of the contractile ring is orchestrated by the small GTPase RhoA. During late anaphase, the centralspindlin complex at the cell center activates the RhoA guanine nucleotide exchange factor (GEF) called ECT2, which in turn activates RhoA at the equatorial cortex. Active RhoA recruits formins — specifically mDia2 — which nucleate unbranched actin filaments, and also activates Rho-kinase (ROCK), which phosphorylates the myosin II regulatory light chain, stimulating myosin motor activity and contractile ring constriction.

Positioning the Cleavage Furrow

The position of the cleavage furrow is determined by signals from two spindle structures: the central spindle (the bundle of antiparallel microtubules between the separating chromosomes) and the astral microtubules (which radiate from the spindle poles to the cell cortex). Together, these structures deliver the RhoA activation signal to the equatorial cortex, ensuring the furrow forms at the correct location midway between the two sets of chromosomes.

Abscission and ESCRT-III

The final step of cytokinesis — abscission — involves the severing of the narrow intercellular bridge that remains after furrow ingression. This process requires the ESCRT-III complex (endosomal sorting complexes required for transport), which assembles as helical filament spirals inside the bridge. ESCRT-III constricts the bridge from within and, with the help of the ATPase VPS4, catalyzes membrane scission to produce two fully separated daughter cells.

“The position of the cleavage furrow is determined by the central spindle and astral microtubules, which send signals to the cell cortex specifying where the contractile ring should assemble.”

Animal vs Plant Cytokinesis