What is Cytokinesis? – Definition, Process & Importance

Cytokinesis is one of the most significant steps that occurs during the last phase of cell division. Cytokinesis means the division of the cytoplasm of parental cells into two daughter cells. There are some distinct features present in the cytokinesis in animal cells and cytokinesis in plant cells. The partitioning of cytoplasm during meiosis and related sexual reproduction also act to determine the fate of the resulting daughter cells.

In this article, we will study the definition of cytokinesis, cytokinesis in animal and plant cells, how cytokinesis is regulated, the disorders that occur due to cytokinesis, and the significance of cytokinesis.

What is Cytokinesis?

Definition of Cytokinesis: Cytokinesis is the division of the cytoplasm of the parent cell into two daughter cells at the end of mitosis or meiosis.

Mitosis accomplishes not only the segregation of duplicated chromosomes into two daughter nuclei (karyokinesis), but the cell itself is divided into two daughter cells by the separation of cytoplasm called cytokinesis at the end of which cell division gets completed. After the process of karyokinesis in telophase, two nuclei are formed in a cell. Now, we have to divide this nucleus into a complete cell and the process of cytokinesis starts from here.

Cytokinesis has 4 steps:

1. Initiation
2. Contraction
3. Membrane insertion
4. Completion.

Cytokinesis Diagram

The labeled diagram of cytokinesis is given below:

Cytokinesis in Animal Cells

Cytokinesis process begins in anaphase and ends in telophase in animal cells because they lack cell walls. It has the following stages:

• Formation of the Contractile Ring: In animal cells, the first step is the formation of a contractile ring composed of actin and myosin filaments. Its positioning is guided by central spindle microtubules, playing a crucial role in centering the actomyosin contractile ring within the cell.
• Cleavage Furrow Regression : As a result of the movement of actin and myosin filaments, a cleavage furrow is formed, which later leads to the construction of a deep cleavage furrow.
• Formation of Midbody: An intracellular bridge is formed between the two separating daughter cells with a specialized transient structure, which is called the mid-body. This mid-body is made up of tightly bundled anti-parallel microtubules that connect these two daughter cells at the end of cytokinesis in animal cells.
• Abscission : The last phase is abscission, which produces two daughter cells. Numerous protein complexes required for transport, such as ESCRT-3 (endosomal sorting complex), aid in position and abcission. These filaments surround the mid-body and contribute to the abscission process. So, the cell division ends with the cutting of microtubules, and this intracellular bridge unites the two daughter cells in exact positions designated by the mid-body.

This complete process of cytokinesis in animal cell can be summarised in 4 phases. These are:

Phases	Explanation
Initiation	The contractile ring initiates and starts to build a cleavage furrow. This happens in the anaphase.
Contraction	As telophase starts and anaphase ends, the contractile ring keeps contracting and widening the cleavage furrow.
Membrane insertion	The process of inserting a newly produced cell membrane between two newly forming cells is known as membrane insertion.
Completion	The contractile ring closes and divides the two new cells from one another at the point of completion.

Cytokinesis in Plant Cells

Since a plant cell contains a cell wall, cytokinesis starts during interphase and terminates in telophase. It has the following stages:

• Phragmoplast Formation: Between two chromosomes, a rearrangement of microtubules occur which leads to the formation of a microtubule-containing structure, phragmoplast, which guides the production of new cell wall.
• Initiation of Cell plate formation: The cell organelle Golgi body is responsible for bringing cell wall materials near the center of the metaphase plate.
• Cell plate formation: The Golgi-vesicles all fuse together to form a continuous membranous structure called the cell plate.
• Cell plate expansion: The cell plate starts expanding towards the outer area with the help of microtubules.
• Fusion with cell membrane: The cell plate then finally fuses with the cell membrane, thus resulting in the division of the cell. After the cell plate divides the cell, the plasma membrane closes, separating the two daughter cells. Trapped endoplasmic reticulum forms the plasmodesmata, or space, between the two cells, allowing chemicals to move from one to the other and signaling between cells.

Regulation of Cytokinesis

There are multiple ways by which cytokinesis is regulated. Some of these are:

1. By Protein Kinases: Multiple mitotic protein kinases are involved in the regulation of cytokinesis. CDKs or cyclin dependent kinase, Polo Kinase (Plk1) and Aurora B kinase complex are few examples of the protein kinases. CDK does not allow cytokinesis to take place until anaphase by phosphorylating cytokinesis components. Plk1 and Aurora B kinase positively regulate the cytokinesis mechanism and are active while CDK1 becomes inactive.
2. By Tyrosine Kinases: Some receptor tyrosine kinases help in regulating cytokinesis regulating key signalling pathways that are involved in cell division. Some of these kinases help in activating downstream cascade reactions that affect the activity of cell cycle regulators.
3. By Lipids: Lipids such as sphingolipids and phospholipids are main constituents of the cell membrane. These lipids are a big part of membrane remodeling process such as membrane curvature generation and membrane scission that eventually result in the cleavage formation, which further leads to cytokinesis. The lipids also act as signaling molecules like phosphatidylinositol 4,5-bisphosphate helps in recruiting proteins which are important for cytokinesis.

Disorders and Abnormalities in Cytokinesis

The following points highlight the abnormalities seen during cytokinesis:

• Abscission, the final phase of cytokinesis, is the separation of cytoplasmic components from one another. Failure at this stage may result in cleavage furrow regression or the establishment of a permanent link between the two daughter cells.
• Cytokine dysregulation causes a variety of human ailments, including Lowe syndrome, blood disorder, cancer and female infertility. Cytokinesis failure can cause centrosome amplification, tetraploid cells, genetic instability, and apoptosis.
• Tetraploid cells are caused by cytokinesis failure, which occurs when diploid, mononucleated cells undergo mitosis but do not form a contractile ring.
• Anaphase spindle elongation does not occur in cells, possibly due to defective actin cytoskeleton reorganization.
• Cytokinesis failure occurs in cells with mutations in the APC (Adenomatous Polyposis Coli) tumour suppressor. Some APC mutations may cause cytokinesis failure by interfering with the microtubule-dependent anchoring of the mitotic spindle.
• Myosin II is the primary motor protein necessary for cytokinesis. Since myosin motor activity is essential for furrow ingression, disrupting myosin localization or activity may result in cytokinesis failure.

Significance of Cytokinesis

The following points help in understanding the importance of cytokinesis:

• Formation of daughter cells: The splitting of the original parent cell into daughter cells occurs with the help of cytokinesis.
• Maintenance of cell size: Cytokinesis helps in the proper distribution of cellular components between the daughter cells.
• Genetic stability: Any kind of irregularity seen during cytokinesis leads to genetic instablity.
• Tissue growth and repair: In case of any damage to the cells, cytokinesis helps in the repair process by forming new cells.
• Development and Differentiation: Cytokinesis helps in the formation of special tissues which thereby help in cell development and differentiation.
• Cellular Homeostasis: A balance is maintained in the body as cytokinesis maintains a proper order between cell growth and cell death.

Conclusion – Cytokinesis

In cytokinesis summary – It is a division of cytoplasm of parent cell into two daughter cells. The process of cytokinesis illustrates the cytoskeleton’s strength and flexibility. In animals, the cleavage furrow is a complex yet dynamic organization of many substances that makes precise characterization difficult. This also applies to division machinery in plants. Cytokinesis is not easily replicated in a test tube. Cytokinesis does not occur sequentially after chromosomal segregation during the cell cycle development. The cell cycle is regulated by various signals, some of which are unique to each organism. Defining cytokinesis will continue to be a valuable research topic.

FAQs on Cytokinesis

What are the 4 Stages of Cytokinesis?

Cytokinesis can be considered to occur in four stages—initiation, contraction, membrane insertion, and completion.

What is Meant by Karyokinesis?

Karyokinesis is the process of dividing a cell’s nucleus to from to nuclei for the two daughter cells during cell division.

Is Cytokinesis in animal Cells Centipetal?

Yes, the cytokinesis in animal cells is centipetal in nature.

What occurs during Cytokinesis?

During cytokinesis, the cell membrane pinches in at the cell equator, forming a cleft called the cleavage furrow.

What Stage occurs after Cytokinesis?

After cytokinesis takes place, the cell cycle restarts. In interphase, the cell prepares for cell division.

What are the Two Types of Cytokinesis?

The two types of cytokinesis are – cytokinesis in plant cell which occurs with the formation of cell plate and other is cytokinesis in animal cell that is embryonic cleavage.

What is Cytokinesis Vs Telophase?

Cytokinesis happens immediately after telophase. The chromosomes move to opposing ends and the nucleus develops around them in telophase and in cytokinesis, the cytoplasm divides alongside the organelle.

Cytokinesis Begins in which Phase?

Cytokinesis starts in the Anaphase phase.