Cytokinesis is the process by which a single eukaryotic cell divides its cytoplasm to form two distinct daughter cells. This physical separation is the final stage of cell division, following the division of the nucleus, known as mitosis. While often discussed together, cytokinesis is a separate yet coordinated event that ensures the successful distribution of cellular components into the newly formed cells. It is a regulated mechanism, allowing for the creation of new cells from a parent cell.
Cytokinesis in Animal Cells
In animal cells, cytokinesis is accomplished through cleavage. This involves the formation of a contractile ring, which assembles beneath the plasma membrane in the cell’s equatorial region. The contractile ring is composed of filamentous actin (F-actin) and the motor protein myosin II. These proteins generate the force for cell constriction.
As the contractile ring tightens, it pulls the cell membrane inward, creating an indentation called a cleavage furrow. This furrow deepens progressively. The continuous contraction of the actin and myosin filaments ultimately pinches the parent cell into two daughter cells, each containing its own nucleus and a portion of the cytoplasm.
Cytokinesis in Plant Cells
Plant cells undergo a distinct mechanism of cytokinesis compared to animal cells, due to their rigid cell wall. Instead of forming a contractile ring and cleavage furrow, plant cells construct a new cell wall in the middle of the dividing cell. This process begins with the formation of a structure called the phragmoplast.
The phragmoplast is an assembly of microtubules and actin filaments that forms between the two newly separated nuclei during late anaphase or telophase. Vesicles from the Golgi apparatus are then guided by the phragmoplast to the equatorial plane of the cell. These vesicles, carrying cell wall materials, fuse to form a flattened, disc-like structure known as the cell plate. The cell plate expands outward until it fuses with the existing parental cell wall, dividing the cell into two daughter cells, each with its own complete cell wall.
The Importance of Cytokinesis
The accurate completion of cytokinesis is important for the functioning and survival of multicellular organisms. Without effective cytoplasmic division, cells would become multinucleated, meaning they would contain multiple nuclei within a single, undivided cytoplasm. This condition leads to abnormal cell function, impaired growth, and can contribute to the development of diseases.
Cytokinesis plays a role in growth, allowing for the increase in cell number for an organism to develop and enlarge. It is also involved in tissue repair, facilitating the replacement of damaged or old cells. Cytokinesis maintains genetic stability and cellular integrity across generations.