Cytokinesis is the crucial stage of cell division where the cytoplasm divides to form two daughter cells. In animal cells, a contractile ring constricts the cell membrane, while plant cells form a cell plate. Understanding cytokinesis is vital for comprehending the process of mitosis. Join us as we explore the stages, differences, and significance of cytokinesis in this comprehensive blog post.

What is Mitosis? A Recap

Mitosis is the process whereby a cell reproduces asexually to produce two identical daughter cells. This occurs via nuclear division. Before mitosis, the cell doubles the number of chromosomes via chromosome replication. This conserves the chromosomal number throughout cell replication; if this did not occur, a diploid cell would divide to produce a haploid cell, which would later divide to contain only half the number of chromosomes, and so forth. Mitosis is important for cell replacement; the development of embryos after sexual reproduction; the growth of organisms after this stage; and asexual reproduction. Mitosis consists of five main phases: prophase, prometaphase, metaphase, anaphase, and telophase. Mitotic events are separated from each other by interphase, during which a cell is not in the process of dividing.

Phases of Mitosis

Prophase involves the migration of centrioles from the centrosome, just outside of the nucleus, to opposite ends of the cell. Remember that plant and fungi cells do not contain centrioles so this only occurs in animal cells. While this is occurring, the nuclear envelope and nucleolus within begin to disintegrate. This allows the chromatin within to separate, thus forming the genetic basis of the two daughter cells. The chromatin condenses, and chromosomes become visible.

In prometaphase, the spindle fibers attach to the kinetochore at the center of each chromosome in preparation to separate the sister chromatids. The chromosomes align on the equatorial plane of the cell. In metaphase, the chromosomes are all aligned on this plane, with the arms of a sister chromatid facing each pole.

Anaphase results in the disjunction of the chromosomes into sister chromatids. They are then pulled to opposite poles via the shortening of spindle fibers. This continues into telophase until there is a diploid number of chromosomes at each pole of the cell.

The entirety of mitosis up until this point is karyokinesis, which refers to the splitting of the nuclear material into genetically identical diploid sets. The cell then splits to form two genetically identical daughter cells, in a process known as cytokinesis. The end of cytokinesis denotes the end of telophase.

What is Cytokinesis?

Cytokinesis is the process where the cytoplasm of a parent cell is divided between two daughter cells. This is also often known as cytoplasmic division or cell cleavage. Cytokinesis begins in anaphase in animal cells and prophase in plant cells. It terminates in telophase in both, to form the two daughter cells produced by mitosis. In essence, cytokinesis is the partitioning of the cytoplasm into two equal parts. Each contains a diploid chromosomal set identical to that of the parent cell.

Once this cytoplasmic material is divided, a plasma membrane (cell membrane) is formed around each new cell. Organelles within the cytoplasm form through replication or synthesis. Since cytoplasmic material is not doubled in mitosis, the daughter cells are about half the volume of the parent cell. Meanwhile, the nucleus of each daughter cell is roughly the same size as that of the parent cell. This is due to the chromosome replication which occurs before mitosis. Cytokinesis takes place in four stages: initiation, contraction, membrane insertion, and completion. The events occurring within these stages differ in animal and plant cells.

Image Source: Wikimedia Commons

Figure 1: Cytokinesis occurs in the late telophase of mitosis in an animal cell.

Cytokinesis in Animal Cells

In animals, the cytoplasm is constricted until the constriction becomes so tight that the two daughter cells are formed. This begins with the formation of a cell furrow or cleavage furrow, a puckering in the cell membrane enclosing the genetic material and cytoplasm. The contractile ring causes this puckering. It is located just below the surface of the cell membrane. The contractile ring initially forms at a point surrounding the equatorial plane of the cell; it then spreads around the circumference of the cell until the whole cell is encircled by a small furrow. This furrow then deepens as the contractile ring contracts.

The cell produces additional membrane material through vesicle fusion to account for the increase in surface area that the cell membrane must enclose and inserts it next to the contractile ring. The contractile ring contracts until it separates the two daughter cells by a midbody, a narrowed portion of the cytoplasm containing the remains of the mitotic spindle. It then breaks off to form two daughter cells that are completely enclosed in separate cell membranes. After cytokinesis, the organelles reassemble in each daughter cell. Some of these, such as mitochondria and chloroplasts, replicate from existing bodies. Others, such as the endoplasmic reticulum and Golgi apparatus, fragment when the nuclear envelope of the parent cell disintegrates, and these fragments regenerate whole organelles in the respective daughter cells.

How Does the Contractile Ring Work?

The spindle fibers or mitotic spindle control the function and position of the contractile ring, which is the same mechanism that controls the movement of chromosomes in mitosis. The alignment of the mitotic spindle is perpendicular to the equatorial plane; the mitotic spindle stretches between the two diploid sets of chromosomes located at the poles of the cell, with the equatorial plane between the poles. This is to prevent incorrect separation of the chromosomes during cytokinesis.

The actin and myosin II fuel the contraction of the contractile ring, in a chemical reaction similar to that which occurs in smooth muscle. The microtubules stabilize the cell furrow as it increases in depth and breadth. The contractile ring dissipates once cleavage is complete.

Image Source: Wikimedia Commons

Figure 2: The mitotic spindle drives the contractile ring creation and positioning in animal cells.

Cytokinesis in Plant Cells

In plants, cytokinesis begins with the production of a cell plate on the equatorial plane. This will later become the middle lamella between the two plant cells. Also, the primary and secondary cell walls of each daughter cell are deposited on either side of the cell plate. This forms the basis of the separation between the two daughter cells.

Cytokinesis is initiated in prophase when a cytoskeleton of actin filaments and microtubules forms around the cell. This band is known as the preprophase band, and will later determine the positioning of the cell plate. The preprophase band is only present in prophase and prometaphase and disintegrates before mitosis reaches the metaphase stage.

The cell plate begins to form in anaphase and is controlled by the phragmoplast, which contains the remains of the mitotic spindle. Microtubules within the mitotic spindle transport vesicles containing polysaccharides and glycoproteins to the center of the phragmoplast, where they fuse to form the early cell plate. This early cell plate continues to expand until it reaches the parent cell membrane and cell wall, where it fuses with the cell wall. The location of this fusion is determined by the location of the preprophase band. Cellulose is later laid down within the early cell plate to create a cell wall, which will divide the parent cell into two daughter cells of equal volume and each containing a diploid set of chromosomes. These can increase in size via later expansion or growth.

Please make sure to keep track of the differences between cytokinesis in plants and cytokinesis in animals. You might find it useful to tie these back to the differences in their cells.

Image Source: Wikimedia Commons

Figure 3: Cytokinesis in plant cells occurs from the inside-out, with the creation of a cell plate.

When Does Cytokinesis Occur?

In animal cells, cytokinesis begins in anaphase, with the mitotic spindle determining the starting position of the contractile ring to form. In telophase, this ring becomes active, and the cleavage furrow forms and deepens until only a thin attachment, the midbody, remains. Cleavage concludes and cytokinesis ends.

In plant cells, cytokinesis begins in prophase, with the production of a cytoskeleton known as a preprophase band. This band disappears before metaphase but is the point at which the new cell wall joins the parent cell wall. A cell plate is deposited at the center of the parent cell, between the two sets of diploid chromosomes. This begins in anaphase. The mitotic spindle, contained within the phragmoplast, controls the creation of this wall from vesicles. The wall continues to grow until it reaches the point in the parent cell wall determined by the preprophase band, forming two complete cells. Cytokinesis ends at the end of telophase.

When Things Go Wrong

Cytokinesis needs to occur at the right time and place. This ensures that each daughter cell has a complete diploid set of chromosomes, and that chromosome movement proceeds without interruption. If cytokinesis occurs at the wrong time or in the wrong place, it can lead to cells with an abnormal amount of chromosomes. This leads to conditions such as aneuploidy, where a cell has several extra or missing chromosomes; polyploidy, where cytokinesis does not occur and a cell has more than a diploid set of chromosomes; or multinucleated cells, where more than one nucleus is present in a cell. These can lead to genetic disorders or even cancers.

Cytokinesis does not occur in the process of mitosis leading to multinucleate cells. In this process, cytokinesis is skipped in favor of rapid development. This occurs in Drosophila fruit flies, embryos, as well as in certain types of mammalian cells.

In some occasions, the location of the contractile ring in animal cells and the cell plate in plant cells is not centered. This leads to the creation of cells of unequal volumes or unequal cytoplasmic content. This is controlled through the movement of the mitotic spindle and is used to create cells that will serve different functions, such as the division of a fertilized egg cell in meiosis.

In conclusion, cytokinesis is central to mitosis understanding this process is essential to fully appreciating biology.

Looking for more Biology practice?

Check out our other articles on Biology.

You can also find thousands of practice questions on Albert.io. Albert.io lets you customize your learning experience to target the practice where you need the most help. We’ll give you challenging practice questions to help you achieve mastery in Biology.

Start practicing here.

Are you a teacher or administrator interested in boosting Biology student outcomes?

Learn more about our school licenses here.