Cytokinesis- Definition and Process (in animal and plant cells)

Cytokinesis is a fascinating and essential process of cell biology that occurs in both plants and animals. It is the final step of cell division, in which the cytoplasm and its contents are split into two new cells. Cytokinesis ensures that each daughter cell receives a complete set of chromosomes, organelles, and other cellular components. Without cytokinesis, cells would become too large and inefficient, or they would end up with abnormal numbers of chromosomes, which can cause diseases such as cancer.

Cytokinesis usually occurs after mitosis, the division of the nucleus and the chromosomes. However, cytokinesis is not a part of mitosis itself, but a separate process that can be regulated independently. Cytokinesis can also occur without mitosis, such as in some unicellular organisms that reproduce by binary fission. In this case, cytokinesis is the main mode of reproduction.

Cytokinesis involves different mechanisms in animal and plant cells, due to their different structures and properties. Animal cells use a contractile ring of actin and myosin filaments to pinch the cell membrane into two. Plant cells use vesicles from the Golgi apparatus to form a cell plate that grows into a new cell wall. Both processes require coordination with the mitotic spindle, which helps to position the plane of division and to segregate the chromosomes.

Cytokinesis is a complex and dynamic process that involves many proteins and molecular interactions. It is also influenced by external signals and environmental factors. Cytokinesis is tightly regulated by checkpoints and feedback mechanisms that ensure its accuracy and timing. Errors or failures in cytokinesis can have serious consequences for cell function and development.

Cytokinesis is the final step of cell division that separates the cytoplasm and the cell membrane of a parent cell into two daughter cells. Cytokinesis usually occurs after mitosis or meiosis, when the chromosomes have been segregated and distributed to the opposite poles of the cell. Cytokinesis ensures that each daughter cell receives a complete set of chromosomes as well as a share of the cytoplasmic components, such as organelles, proteins, and metabolites.

Cytokinesis is a physical process that involves the formation of a contractile ring in animal cells or a cell plate in plant cells. The contractile ring is composed of actin and myosin filaments that constrict the cell membrane and create a cleavage furrow that pinches off the cell into two. The cell plate is formed by the fusion of vesicles derived from the Golgi apparatus that carry cell wall materials to the middle of the cell. The cell plate grows outward and eventually fuses with the existing cell wall, creating a new cell wall between the daughter cells.

Cytokinesis is regulated by various factors, such as signals from the mitotic spindle, the position of the chromosomes, and the availability of energy and nutrients. Cytokinesis can also be influenced by external cues, such as mechanical stress, cell-cell interactions, and environmental conditions. Cytokinesis is essential for growth, development, tissue repair, and reproduction in multicellular organisms. Cytokinesis also plays a role in cellular differentiation, as some cells undergo asymmetric cytokinesis to produce daughter cells with different fates.

Cytokinesis can be divided into four stages: initiation, contraction or expansion, abscission or fusion, and completion. Initiation is when the cleavage furrow or the cell plate starts to form at the equator of the cell. Contraction or expansion is when the contractile ring or the cell plate tightens or widens to separate the cytoplasm. Abscission or fusion is when the plasma membrane or the cell wall is cut or joined to form two distinct cells. Completion is when the daughter cells are fully separated and begin their own cell cycle.

Cytokinesis can be distinguished from karyokinesis, which is the division of the nucleus and the chromosomes. Karyokinesis and cytokinesis are often coordinated and synchronized, but they can also occur independently or at different times. For example, some cells undergo multiple rounds of karyokinesis without cytokinesis, resulting in multinucleated cells. Some cells undergo cytokinesis without karyokinesis, resulting in anucleated cells. Some cells undergo karyokinesis and cytokinesis at different rates, resulting in unequal distribution of chromosomes or cytoplasm.

Cytokinesis is a complex and dynamic process that requires coordination and regulation of various molecular and cellular mechanisms. Cytokinesis is also an important subject of research, as defects or abnormalities in cytokinesis can lead to diseases such as cancer, polyploidy, aneuploidy, or infertility.

Cytokinesis is the physical division of the cell cytoplasm, the cell membrane, and cell organelles in eukaryotic cells to produce two distinct cells at the end of the cell cycle in both mitosis and meiosis. Cytokinesis usually occurs after the nuclear division (mitosis or meiosis), but in some cases, it may happen without nuclear division (such as in multinucleated cells).

Cytokinesis differs between animal and plant cells due to the presence or absence of a rigid cell wall. Animal cells have a flexible plasma membrane that can be pinched inward by a contractile ring of microfilaments, while plant cells have a stiff cell wall that requires the formation of a new cell plate between the daughter cells.

Cytokinesis in animal cells

In animal cells, cytokinesis begins during anaphase or telophase of mitosis, when a contractile ring of actin and myosin filaments forms under the plasma membrane at the equator of the cell. The contractile ring acts like a purse string, pulling the membrane inward and creating a cleavage furrow that deepens until it reaches the center of the cell. The cleavage furrow divides the cytoplasm and the organelles into two roughly equal halves, each containing one nucleus.

The contractile ring is guided by signals from the mitotic spindle, which determines the plane of cell division. The spindle microtubules also form a structure called the midbody at the center of the cleavage furrow, which contains proteins that help sever and fuse the plasma membrane. The midbody may persist as a cytoplasmic bridge between the daughter cells until it is degraded or absorbed.

Cytokinesis in plant cells

In plant cells, cytokinesis also begins during anaphase or telophase of mitosis, but instead of a contractile ring, a cell plate is formed by the fusion of vesicles from the Golgi apparatus. The vesicles carry cell wall materials such as cellulose and pectin, and they align at the equator of the cell along the remnants of the mitotic spindle, which form a structure called the phragmoplast. The cell plate grows outward from the center of the cell until it reaches and fuses with the existing cell wall at the edges. The cell plate divides the cytoplasm and the organelles into two separate compartments, each containing one nucleus.

The phragmoplast is independent of the spindle microtubules and does not determine the plane of cell division. Instead, other factors such as pre-existing cell walls, cortical microtubules, and cytoskeletal elements may influence the orientation of cytokinesis in plant cells. The phragmoplast also facilitates the transport and delivery of vesicles to the cell plate.

Microtubules are protein filaments that form part of the cytoskeleton and play a crucial role in cell division. They are involved in both the formation and the function of the mitotic spindle, which is a structure that separates the chromosomes during mitosis. Microtubules also determine the position and orientation of the cleavage furrow, which is the indentation that forms on the cell surface during cytokinesis.

In animal cells, cytokinesis is achieved by the contraction of a ring of actin and myosin filaments, called the contractile ring, that forms under the plasma membrane at the site of the cleavage furrow. The contractile ring is positioned and stabilized by a bundle of microtubules that extends from the spindle poles to the equator of the cell, called the central spindle. The central spindle also serves as a platform for the recruitment and activation of proteins that regulate cytokinesis, such as RhoA, Aurora B kinase, and Cytokinesis A (Cyk4).

In plant cells, cytokinesis is achieved by the formation of a new cell wall between the daughter cells, called the cell plate. The cell plate is derived from vesicles that are transported along microtubules from the Golgi apparatus to the center of the cell. These vesicles fuse together to form a disc-shaped structure that expands outward until it reaches and fuses with the existing cell wall. The microtubules that guide the vesicles and support the cell plate are organized into a structure called the phragmoplast, which consists of two parallel arrays of microtubules that converge at the equator of the cell.

Cell organelles, such as mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes, are also distributed to the daughter cells during cytokinesis. This process requires coordination between microtubules and motor proteins that transport organelles along them. For instance, mitochondria are captured by microtubules near the spindle poles and then moved to the periphery of the cell by kinesin motors. Endoplasmic reticulum and Golgi apparatus are partitioned by being pulled along microtubules by dynein motors. Lysosomes are segregated by being attached to both actin and microtubule filaments.

Thus, microtubules and cell organelles play important roles in cytokinesis by ensuring that each daughter cell receives a complete set of genetic material, cytoplasmic components, and membrane structures.

Cytokinesis is the process of dividing the cytoplasm and the cell membrane of a parent cell into two daughter cells. Cytokinesis usually occurs after mitosis, when the chromosomes have been separated into two sets. Cytokinesis can be divided into four main stages: initiation, contraction, membrane insertion, and completion.

• Initiation: This stage begins during anaphase or telophase of mitosis, when the cell starts to form a cleavage furrow (in animal cells) or a cell plate (in plant cells). The cleavage furrow is a shallow groove on the cell surface that marks the site of cell division. The cell plate is a disk-shaped structure that forms in the middle of the cell and grows outward. Both the cleavage furrow and the cell plate are guided by the position of the mitotic spindle, which consists of microtubules that attach to the chromosomes and pull them apart.
• Contraction: This stage involves the tightening of the cleavage furrow or the expansion of the cell plate. In animal cells, the cleavage furrow is constricted by a contractile ring of actin and myosin filaments, which are proteins that generate force and movement. The contractile ring pinches the cell membrane and cytoplasm into two parts. In plant cells, the cell plate is enlarged by the fusion of vesicles from the Golgi apparatus, which carry cell wall materials such as cellulose and pectin. The vesicles join together at the center of the cell and form a new cell wall between the two daughter cells.
• Membrane insertion: This stage involves the addition of new membrane components to seal off the daughter cells. In animal cells, this is done by the fusion of intracellular vesicles with the plasma membrane at the edges of the cleavage furrow. The vesicles provide lipids and proteins that are needed for membrane growth and function. In plant cells, this is done by the incorporation of Golgi membranes into the plasma membrane at the periphery of the cell plate. The Golgi membranes also supply enzymes and structural proteins that are needed for cell wall synthesis and maturation.
• Completion: This stage marks the end of cytokinesis and the formation of two fully developed daughter cells. In animal cells, this is achieved by the final separation of the cytoplasm and organelles, as well as the dissolution of any remaining connections between the cells, such as midbody structures or gap junctions. Midbody structures are remnants of microtubules that span across the cleavage furrow and help coordinate its closure. Gap junctions are channels of endoplasmic reticulum that allow communication and transport between adjacent cells. In plant cells, this is achieved by the completion of cell wall formation and differentiation, as well as the establishment of plasmodesmata between neighboring cells. Plasmodesmata are pores in the cell wall that allow communication and transport between adjacent cells.

By following these four stages, cytokinesis ensures that each daughter cell receives a complete set of chromosomes, a sufficient amount of cytoplasm and organelles, and a functional cell membrane and cell wall (if present). Cytokinesis is essential for growth, development, repair, and reproduction in eukaryotic organisms.

Cytokinesis is the process of dividing the cytoplasm and the cell membrane of a parent cell into two daughter cells. Although cytokinesis is similar in both animal and plant cells, there are some key differences due to the presence of a rigid cell wall in plant cells. Here are some of the main differences between cytokinesis in animal and plant cells:

• Timing: Cytokinesis begins in anaphase in animal cells, when the chromosomes are separated and moved to opposite poles of the cell. In plant cells, cytokinesis begins in prophase, when the nuclear envelope breaks down and the spindle apparatus forms.
• Mechanism: Animal cells use a contractile ring of actin and myosin filaments to form a cleavage furrow that pinches the cell membrane into two. Plant cells use vesicles from the Golgi apparatus to form a cell plate that grows outward from the center of the cell until it reaches the existing cell wall.
• Outcome: Animal cells produce two separate daughter cells that may remain connected by cytoplasmic bridges or gap junctions. Plant cells produce two daughter cells that are separated by a new cell wall, but may communicate through plasmodesmata.
• Variation: Animal cells may undergo symmetrical or asymmetrical cytokinesis, depending on the distribution of cytoplasm and organelles between the daughter cells. Plant cells usually undergo symmetrical cytokinesis, except for some specialized cases such as pollen formation.

As mentioned earlier, plant cells have a rigid cell wall that prevents the formation of a cleavage furrow as in animal cells. Therefore, plant cells use a different mechanism to divide their cytoplasm and form a new cell wall between the daughter cells. This mechanism involves the formation of a phragmoplast, which is a plant-specific structure that only appears during late cytokinesis.

A phragmoplast is a complex assembly of microtubules, microfilaments, endoplasmic reticulum and Golgi-derived vesicles that forms between the two daughter nuclei after the separation of chromosomes. The phragmoplast has a barrel-shaped appearance and consists of two opposing sets of microtubules that interdigitate at the equator, where the cell plate will form.

The phragmoplast acts as a scaffold and a guide for the delivery and fusion of vesicles that carry cell wall components to the site of cell plate formation. The vesicles are thought to be transported along the phragmoplast microtubules by a plus-end-directed kinesin-like motor protein, which has been identified in some plant species.

The cell plate is initiated as a disc of fused vesicles at the center of the phragmoplast, and then expands outward as more vesicles are added to its edges. As the cell plate grows, the phragmoplast also expands by adding new microtubules at its periphery and removing old ones at its center. This ensures that the phragmoplast and the cell plate remain aligned and reach the parent cell wall at the same position where the preprophase band was located.

The phragmoplast also plays a role in the incorporation of endoplasmic reticulum and plasma membrane into the cell plate. Some segments of smooth endoplasmic reticulum are trapped within the cell plate as it forms, and later become the plasmodesmata that connect the two daughter cells. The Golgi membranes that surround the vesicles are also integrated into the plasma membrane of the new cell wall.

The final step of cytokinesis in plant cells is the completion of the cell wall by adding cellulose and other polysaccharides to the cell plate. The cellulose is synthesized by enzymes that are carried by the phragmoplast microtubules to the cell plate. The cellulose molecules interact and form a strong and rigid matrix that gives stability and structure to the plant cell wall.

The phragmoplast is thus an essential structure for plant cytokinesis, as it enables the formation of a new cell wall between two daughter cells without compromising their integrity and communication. The phragmoplast is also a unique feature of plant cells, as it is not found in any other eukaryotic group.

Cytokinesis research has many applications in various fields of biology and medicine. Some of the applications are:

• Cytokinesis research can help to understand the mechanisms of cell division and differentiation, which are essential for normal development, tissue homeostasis and wound healing.
• Cytokinesis research can also reveal the causes and consequences of abnormal cell division, such as aneuploidy, polyploidy, multinucleation and cytokinesis failure, which are associated with various diseases, such as cancer, infertility and developmental disorders.
• Cytokinesis research can provide insights into the evolution of cell division mechanisms and their diversity among different organisms and cell types.
• Cytokinesis research can enable the development of new tools and techniques for studying cell biology, such as live-cell imaging, optogenetics, micromanipulation and computational modeling.
• Cytokinesis research can also facilitate the discovery of new targets and strategies for therapeutic intervention, such as drugs that modulate cytokinesis or prevent its completion in cancer cells.

Some examples of cytokinesis research applications are:

• The cytokinesis-block micronucleus assay (CBMN) is a simple and reliable method for assessing genome damage in human peripheral blood lymphocytes. It can be used to monitor the effects of environmental factors, such as radiation, chemicals and radon exposure, on human health.
• The block-cytokinesis micronuclei cytosome assay (BCMCA) is a modified version of the CBMN assay that can measure both chromosome damage and nuclear division defects in human lymphocytes. It can be used to study the genetic instability and susceptibility to cancer in individuals with different genetic backgrounds or diseases.
• The phragmoplast is a plant-specific structure that mediates cytokinesis in plant cells. It is composed of microtubules, actin filaments and vesicles that fuse to form a cell plate. The phragmoplast is a model system for studying the dynamics and regulation of cytoskeletal elements and membrane trafficking during cytokinesis.

Overall, cytokinesis research has wide-ranging implications and contributes to our understanding of fundamental biological processes and their relevance to human health and disease.

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