Cytokinesis concludes cell division, ensuring the precise distribution of cellular components into new daughter cells. This process involves dividing the cytoplasm, organelles, and cell membrane after the genetic material separates during nuclear division (mitosis or meiosis). Understanding how cytokinesis occurs in animal cells reveals the intricate mechanisms governing cellular reproduction and organismal development.
Cytokinesis: Dividing the Cell’s Contents
After a cell’s nucleus divides, cytokinesis physically separates the cell into two daughter cells. This ensures that the two new nuclei, along with cytoplasm and organelles, are accurately partitioned into each nascent cell. While nuclear division focuses on chromosome segregation, cytokinesis completes cell division by physically cleaving the original cell. This separation ensures each new cell receives the necessary cellular machinery to function independently.
The Contractile Ring: The Key Player
The primary structure responsible for cytokinesis in animal cells is the contractile ring. This dynamic assembly forms just beneath the plasma membrane, at the cell’s equator, perpendicular to the mitotic spindle. The contractile ring is composed of filamentous actin (F-actin) and myosin II motor proteins. These components enable the ring to generate the force required for cellular constriction. Other regulatory proteins also associate with the ring, assisting in its assembly and function.
The Process of Cleavage Furrow Formation
Cleavage furrow formation begins shortly after chromosomes separate during anaphase of mitosis. Signals from the central spindle, a structure formed by microtubules, guide the contractile ring’s precise positioning at the cell’s midplane. Actin and myosin II are then recruited and assembled into a circumferential ring at this equatorial region. This assembly initiates an indentation on the cell’s surface, known as the cleavage furrow.
The contractile ring’s active constriction drives the cleavage furrow’s deepening. Myosin II motor proteins slide along actin filaments, generating a “purse-string” like tightening action that pulls the cell membrane inward. This continuous contraction effectively pinches the cell in two. As the furrow deepens, the ring continuously shrinks, and opposing membranes meet and fuse. This final separation, termed abscission, involves a transient intercellular bridge before the two daughter cells fully detach.
The Significance of Cytokinesis
Accurate cytokinesis is important for producing two genetically identical and functionally separate daughter cells. This process supports various biological functions, including organismal growth and the repair of damaged tissues. Errors during cytokinesis can lead to cells with an abnormal number of chromosomes, a condition known as aneuploidy. Such abnormalities can contribute to the development of certain diseases, such as cancer.