Cytokinesis is the process of physical division of the cytoplasm of a parent cell into two separate daughter cells during cell division. It completes the separation of the daughter cells.
During mitosis, the chromosomes are separated and two daughter nuclei are formed. The division of the nucleus is referred to as karyokinesis. But the formation of two daughter nuclei does not mean that the cell is physically divided into two.
Cytokinesis generally starts during anaphase and continues through telophase. The timing is not same in all cells. Telophase is a stage of mitosis, while cytokinesis is a separate process.
It can also take place during meiosis. The pattern of cytokinesis varies in different organisms.
In animal cells, the cell membrane moves inward and forms a cleavage furrow. This furrow divides the cell into two. In typical plant cells, a cell plate is formed between the daughter nuclei, which later develops into a new partition separating the two daughter cells.
When Does Cytokinesis Occur?
Cytokinesis takes place towards the end of the mitotic phase (M phase) of the cell cycle. In most animal cells, the division of cytoplasm begins during late anaphase, when the chromosomes are moving towards the opposite poles of the cell. It continues through telophase. The process is usually completed near the end of M phase, but in some cells it may extend into the beginning of the next interphase.
During meiosis, cytokinesis can occur after both meiosis I and meiosis II. In many organisms, the cytoplasm is first divided into two cells after meiosis I. Another division takes place after meiosis II, forming four daughter cells. But this pattern is not same in all organisms. In some flowering plants, both nuclear divisions are completed before the division of cytoplasm takes place.
The daughter cells formed after mitotic cytokinesis generally enter the first gap phase (G1 phase) of interphase. In this phase, the cells grow and carry out their normal cellular activities.
Some cells may leave the active cell cycle from G1 and enter a nondividing state called the G0 phase (G zero phase). They may remain in this state temporarily or permanently, depending on the cell type.
Types of Cytokinesis
Cytokinesis is commonly divided into two main types based on the direction in which the cytoplasm is divided. The mechanism is not same in all organisms.
The two main types of cytokinesis are as follows-
1. Centripetal Cytokinesis (Furrow Formation)
This type of cytokinesis is generally found in animal cells. The division starts from the outer surface of the cell and gradually proceeds towards the centre.
An actin-myosin contractile ring is formed beneath the plasma membrane at the equatorial region, which contracts and pulls the membrane inward. This forms a cleavage furrow. The furrow becomes deeper until the two daughter cells remain connected by a narrow intercellular bridge. The final separation takes place by a process called abscission.
2. Centrifugal Cytokinesis (Cell Plate Formation)
In higher plant cells, cytokinesis generally takes place by the formation of a cell plate. The division starts at the central region of the cell and proceeds towards the outer cell wall.
During this process, Golgi-derived vesicles are transported towards the division plane with the help of the phragmoplast, where they fuse to form the developing cell plate. More vesicles are added to the cell plate.
It gradually grows outward and joins the existing plasma membrane and cell wall of the parent cell. The cell plate later develops into a new cell wall between the two daughter cells.
Cytokinesis in Animal Cells
In animal cells, cytokinesis generally takes place by the formation of a cleavage furrow, which divides the cytoplasm of the parent cell into two daughter cells.
The process of cytokinesis in animal cells takes place in the following steps-
- During anaphase, the mitotic spindle helps to determine the position where the cell will divide. The division plane is generally located at the equatorial region of the cell, between the two groups of separating chromosomes.
- At the division site, actin filaments and myosin II proteins assemble beneath the plasma membrane, forming a ring-like structure called the actin-myosin contractile ring. It surrounds the equatorial region of the cell.
- The myosin II proteins interact with the actin filaments and produce a contractile force. This causes the ring to become smaller, pulling the plasma membrane inward. A groove is formed at the equatorial region, which is referred to as the cleavage furrow. As the ring continues to contract, the furrow becomes deeper and the cytoplasm is gradually divided into two portions.
- The cleavage furrow continues to move inward until the two daughter cells remain connected by a narrow cytoplasmic region called the intercellular bridge. Within this bridge, the remaining spindle microtubules are bundled together to form a dense structure known as the midbody.
- In the final step, the narrow connection between the daughter cells is cut by the membrane-remodelling machinery of the cell. This process is called abscission. The remaining connection is broken and two physically separate daughter cells are formed.
Cytokinesis in Plant Cells
In plant cells, cytokinesis generally takes place by the formation of a cell plate at the equatorial region of the parent cell. The rigid cell wall prevents the cell membrane from moving inward as in animal cells.
The process of cytokinesis in plant cells takes place in the following steps-
- Before nuclear division, the future division site is marked in many plant cells by a ring of microtubules called the preprophase band (PPB). This band disappears as mitosis proceeds, but the position of the division site remains marked at the cell cortex.
- During late anaphase and telophase, the remaining spindle microtubules become arranged between the two daughter nuclei. Together with actin filaments and other associated components, they form a structure called the phragmoplast. It provides the framework for the formation of the cell plate.
- Small membrane-bound vesicles derived mainly from the Golgi apparatus are transported towards the middle of the phragmoplast. These vesicles contain membrane components and materials required for the formation of new cell wall. They fuse with each other and form an initial membrane network, which is referred to as the cell plate.
- More vesicles are added to the developing cell plate. The membrane network gradually develops into a sheet-like structure, with cell wall materials deposited within it. Callose, a polysaccharide, is also deposited in the developing cell plate.
- The phragmoplast gradually expands towards the outer region of the cell. During this process, the cell plate also grows outward by the continuous addition and fusion of new vesicles at its edges. The growth continues until the cell plate reaches the existing plasma membrane and cell wall of the parent cell.
- The membranes of the cell plate fuse with the parent cell plasma membrane. The new partition joins the existing cell wall. As the cell plate matures, much of the callose is removed and cellulose along with other cell wall materials are deposited to form the new cell wall between the two daughter cells. Some cytoplasmic connections may remain between the cells in the form of plasmodesmata.
Importance of Cytokinesis
Cytokinesis is the final division of the cytoplasm during cell division, which separates the parent cell into two daughter cells.
The important functions and significance of cytokinesis are as follows-
- Completion of Cell Division- The daughter cells are physically separated after the division of nucleus. In absence of cytokinesis, the nuclei may divide but the cell remains undivided.
- Increase in Cell Number- During mitosis, cytokinesis allows the formation of new cells required for growth and development of multicellular organisms.
- Tissue Repair and Replacement- The dead, damaged and worn-out cells are replaced by new cells formed through cell division. Cytokinesis is involved in the repair and regeneration of tissues where the cells are capable of dividing.
- Distribution of Cellular Components- During cytokinesis, the cytoplasm and different cell organelles are distributed between the daughter cells. They receive the cellular components required for their functions. The distribution may not be equal in all types of cell division.
- Maintenance of Normal Cell Division- In absence of proper cytokinesis, a cell may retain multiple nuclei or a doubled chromosome set. Some cells may become tetraploid and contain extra centrosomes. These cells have an increased chance of chromosome segregation errors during subsequent cell divisions.
- Formation of New Cell Wall- In plant cells, the cell plate formed during cytokinesis later develops into a new cell wall between the daughter cells. The position of this wall also affects the arrangement of cells within the plant tissues.
- Formation of Meiotic Products- Cytokinesis can take place after both meiosis I and meiosis II. In many organisms, the cytoplasm is divided to form separate haploid cells, which may later develop into gametes or spores. The pattern is not same in all organisms.
- Cell Differentiation- In some asymmetric cell divisions, the cytoplasm is divided unequally between the daughter cells. They may receive different cellular components and develop different cell fates.
Failure of Cytokinesis and Its Consequences
Failure of cytokinesis occurs when the nucleus divides but the cytoplasm is not separated into two daughter cells. It may occur due to defects in the contractile ring, incomplete formation of cleavage furrow or failure of the final cell separation (abscission).
The major consequences of cytokinesis failure are as follows-
- Formation of Binucleate Cells- When nuclear division is completed without cytokinesis, the two daughter nuclei remain within a single cell. These cells are referred to as binucleate cells. Repeated failure of cytokinesis may also produce cells containing multiple nuclei.
- Polyploidy- In a normally diploid cell, failure of cytokinesis after mitosis may produce a tetraploid cell (4n), containing four complete sets of chromosomes instead of two. This is a type of polyploidy.
- Extra Centrosomes- Some cells formed after cytokinesis failure may contain extra centrosomes along with the doubled chromosome sets. These centrosomes can produce abnormal mitotic spindle arrangements during the next cell division.
- Chromosome Segregation Errors- The formation of abnormal mitotic spindles in cells with extra centrosomes may cause improper separation of chromosomes. Some daughter cells receive extra chromosomes, while others may lose chromosomes. This abnormal chromosome number is referred to as aneuploidy.
- Cell Cycle Arrest- Some cells stop dividing after failure of cytokinesis. They may remain arrested in the first gap phase (G1 phase) of the cell cycle. The p53 protein may be activated during this response. But the response is not same in all cells. Some cells may continue into another cell cycle.
- Cell Death or Senescence- Cells containing abnormal chromosome contents may undergo programmed cell death (apoptosis). Some may enter a permanent state of cell cycle arrest called cellular senescence.
- Cancer Development- When the abnormal cells continue to divide, they may accumulate chromosome segregation errors and other genetic changes. This can contribute to the development of cancer in some cells. But cytokinesis failure does not always produce cancer.
- Abnormal Plant Cell Division- In plant cells, defective formation of the cell plate may produce incomplete cell walls and multinucleate cells. These abnormalities can affect normal tissue organisation, cell differentiation and organ development.
- Normal Polyploid Cell Formation- In some cells, failure of cytokinesis is a normal part of development. Certain liver cells (hepatocytes) may become binucleate or polyploid due to cytokinesis failure during normal liver development.
Plant Cytokinesis vs. Animal Cytokinesis
Plant cytokinesis generally takes place by the formation of a cell plate, while animal cytokinesis occurs by the formation of a cleavage furrow. The mechanism of cytoplasmic division is different in these two types of cells.
The major differences between plant and animal cytokinesis are as follows-
| Features | Plant Cytokinesis | Animal Cytokinesis |
|---|---|---|
| Occurrence | Generally occurs by cell plate formation in higher plant cells. | Generally occurs by cleavage furrow formation in animal cells. |
| Type of Division | Centrifugal division. The division starts at the centre of the cell and proceeds towards the outer cell wall. | Centripetal division. It starts from the cell surface and proceeds inward. |
| Cell Wall | A rigid cell wall is present. The new partition is formed inside the parent cell. | A rigid cell wall is absent. The plasma membrane moves inward during cytokinesis. |
| Division Plane | In many somatic plant cells, the future division site is marked by the preprophase band before mitosis. | The position of the cleavage furrow is mainly determined by signals from the mitotic spindle during anaphase. |
| Main Structure | The phragmoplast is formed between the daughter nuclei. It contains microtubules, actin filaments and associated components. | An actin-myosin contractile ring is formed beneath the plasma membrane at the equatorial region. |
| Mechanism | Golgi-derived vesicles move towards the division plane and fuse with each other to form the cell plate. | The contractile ring gradually contracts and pulls the plasma membrane inward, forming the cleavage furrow. |
| Role of Vesicles | Vesicle fusion is required for the formation and growth of the cell plate. | Membrane trafficking contributes to furrow formation and final separation, but the cleavage furrow is mainly driven by contractile ring contraction. |
| Direction of Growth | The cell plate grows outward from the central region towards the parental cell wall. | The cleavage furrow gradually becomes deeper towards the central region of the cell. |
| Plasma Membrane | The vesicle membranes form new plasma membrane surfaces on either side of the developing cell plate. | The existing plasma membrane is drawn inward and later separated between the daughter cells. |
| Final Separation | The cell plate joins the parental plasma membrane and cell wall. It later develops into a new cell wall between the daughter cells. | The daughter cells remain connected by an intercellular bridge. The final separation takes place by abscission. |
| Timing | Cell plate formation generally begins during late anaphase or telophase. | Cleavage furrow formation generally begins during anaphase and continues through telophase. Final separation may occur later. |
| Result | Two daughter cells are formed with a new cell wall between them. | Two physically separate daughter cells are formed without a cell wall. |
Cytokinesis vs. Mitosis
Mitosis is the process of nuclear division in which the replicated chromosomes are separated into two daughter nuclei. The division of nucleus is also referred to as karyokinesis. During this process, the chromosomes are arranged and separated with the help of the mitotic spindle.
Cytokinesis is the physical division of the cytoplasm of a parent cell into two daughter cells. It is different from mitosis. The process involves the separation of cytoplasm and its cellular components between the daughter cells.
Both mitosis and cytokinesis take place during the mitotic phase (M phase) of the cell cycle. In many animal cells, cytokinesis begins during anaphase and continues through telophase. The timing is not same in all cells. Mitosis and cytokinesis are separate but coordinated processes, and their events may overlap.
The major differences between mitosis and cytokinesis are as follows-
| Features | Mitosis | Cytokinesis |
|---|---|---|
| Definition | Mitosis is the division of the nucleus into two daughter nuclei. | Cytokinesis is the division of the cytoplasm into two separate daughter cells. |
| Type of Division | Nuclear division (karyokinesis). | Cytoplasmic division. |
| Main Function | Separates the replicated chromosomes into two groups. | Separates the cytoplasm and distributes cellular components between the daughter cells. |
| Cellular Machinery | Mainly involves the mitotic spindle, made up of microtubules and associated proteins. | In animal cells, it involves the actin-myosin contractile ring. In higher plant cells, the phragmoplast helps in cell plate formation. |
| Stages | Includes prophase, prometaphase, metaphase, anaphase and telophase. | Involves cleavage furrow formation and final separation in animal cells, or cell plate formation in higher plant cells. |
| Timing | Generally starts with prophase and ends with telophase. | Often starts during anaphase, overlaps with telophase and may continue into the beginning of interphase. |
| Chromosomes | The sister chromatids are separated and distributed between two daughter nuclei. | Chromosomes are not separated by cytokinesis. |
| Cell Membrane | Physical separation of the plasma membrane is not the main function of mitosis. | The plasma membrane is separated between daughter cells. In higher plant cells, a new cell plate and cell wall are formed. |
| Result | Two daughter nuclei are formed. | Two physically separate daughter cells are generally formed. |
| Failure of Division | Failure of chromosome segregation may produce nuclei with abnormal chromosome numbers. | Failure of cytokinesis may produce binucleate or polyploid cells. |
| Relationship | Mitosis can occur without cytokinesis in some cells. | Cytokinesis generally takes place in coordination with mitosis, but it is not a stage of mitosis. |
Cytokinesis at a Glance
| Feature | Description |
|---|---|
| Definition | Cytokinesis is the physical division of the cytoplasm of a parent cell into separate daughter cells. |
| Cell Cycle Phase | Occurs towards the end of the mitotic phase (M phase). |
| Starting Phase | Generally begins during anaphase in animal cells and overlaps with telophase. |
| Completion | Usually completed near the end of M phase. |
| Karyokinesis vs Cytokinesis | Karyokinesis divides the nucleus, while cytokinesis divides the cytoplasm. |
| Cytokinesis in Animal Cells | Takes place by the formation of a cleavage furrow. |
| Contractile Ring | Made up mainly of actin filaments and myosin II proteins. |
| Cleavage Furrow | An inward groove formed by contraction of the actin-myosin ring. |
| Intercellular Bridge | A narrow cytoplasmic connection that remains between the daughter cells before final separation. |
| Abscission | The final separation of daughter cells by cutting the intercellular bridge. |
| Cytokinesis in Plant Cells | Generally occurs by the formation of a cell plate. |
| Phragmoplast | A structure mainly consisting of microtubules and associated components that guides cell plate formation. |
| Cell Plate Formation | Golgi-derived vesicles fuse at the division site and form the developing cell plate. |
| New Cell Wall | The cell plate develops into a new partition between the daughter cells. |
| Cytokinesis in Meiosis | Can occur after meiosis I and meiosis II. The pattern varies among organisms. |
| Daughter Cells Formed | Usually two after mitotic cytokinesis. Meiosis commonly produces four haploid cells after both divisions. |
| Importance | Involved in cell separation, growth, tissue repair, cell replacement and formation of meiotic products. |
| Failure of Cytokinesis | May produce binucleate, multinucleate or polyploid cells. |
| Possible Consequences | Extra centrosomes, chromosome segregation errors, aneuploidy, cell cycle arrest or cell death. |
| Normal Exceptions | Cytokinesis failure occurs naturally in some cells, such as certain liver cells (hepatocytes), during development. |
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