Cytokinesis – Definition and Process In animal and Plant Cells

What is Cytokinesis? – Cytokinesis Definition

Cytokinesis is the process by which eukaryotic cells divide their cytoplasm into two new, separate cells. Cytokinesis is the final stage of the cell cycle that happens after mitosis or meiosis.

• Cytokinesis is the process by which cells of animals divide; it involves a contractile ring of microfilaments pinching the cell membrane in two. There is a cellular division in plants, where a cell plate is built to separate the two halves.
• In mitosis and meiosis, cytoplasmic division begins in the late phases of nuclear division. The spindle apparatus is responsible for dividing and transporting the duplicated chromatids into the cytoplasm of the daughter cells during cytokinesis.
• Hence, it guarantees that the number and complement of chromosomes will be passed down from one generation to the next and that, barring rare circumstances, the daughter cells will be functioning clones of the parent cell. Each daughter cell enters an interphase of the cell cycle once telophase and cytokinesis are complete.
• In animals, for example, during oogenesis, the developing ovum absorbs nearly all of the cell’s cytoplasm and organelles, deviating significantly from the symmetrical cytokinesis process.
• The resulting polar bodies are left with very little, and in most species they just perish without serving any purpose. But, in other species they take on a wide range of unique roles. The liver and skeletal muscle both have a variant of mitosis that does not include cytokinesis and produces multinucleate cells.
• The stiffness of plant cell walls is one reason why plant cytokinesis is distinct from animal cytokinesis. Instead of a cleavage furrow forming between plant daughter cells like it does in animals, a dividing structure called the cell plate emerges in the cytoplasm and develops into a new, doubled cell wall. The cell is split in half to create two new cells.
• Cytokinesis is very similar to the prokaryotic process of binary fission, but the processes are different due to the changes in cell structure and function between prokaryotic and eukaryotic cells.
• In contrast to the linear, typically numerous chromosomes of eukaryotes, the chromosomes of a bacterial cell are circular (a single chromosome in the form of a closed loop). To that end, bacteria do not build a mitotic spindle prior to cell division.
• Nevertheless, DNA replication occurs during the actual separation of chromosomes in prokaryotes, whereas in mitosis it occurs during the interphase prior to the start of mitosis, even if the daughter chromatids do not fully separate until the anaphase.

What happens during cytokinesis?

• Cytokinesis is the final phase of cell division, occurring after mitosis (division of the nucleus). The cytoplasm of the cell divides into two daughter cells, each with its own nucleus, during cytokinesis.
• In animal cells, a structure known as the contractile ring forms around the cell’s equator and contracts, drawing the plasma membrane inward and forming a cleavage furrow that ultimately divides the cell in half.
• In plant cells, cytokinesis differs slightly. In place of a contractile ring, a cell plate forms in the center of the cell and expands outward until it reaches the cell wall. The cell plate then fuses with the cell wall to form a new cell membrane that divides the daughter cells.
• Overall, cytokinesis guarantees that the genetic material is evenly divided across the two daughter cells, allowing for the continuation of normal cellular processes and growth.

Stages of cytokinesis

• Beginning and development of the cleavage furrow: The initial physical change noticed during cytokinesis is the emergence of the cleavage furrow on the cell surface. The furrow begins to deepen, encircling the cell until it entirely divides in half.
• Reduction and restriction: Likewise known as abscission. The contractile ring, which is composed of actin, myosin, and regulatory proteins, is responsible for cytokinesis in animal cells. This ring is produced beneath the surface of an animal cell during cell division. These rings can compress the cell in half by contracting and constricting it.
• Membrane Insertion: Concurrently, by the fusion of intracellular vesicles, a new membrane is generated and inserted into the cell membrane close to the contractile ring. Once cytoplasmic division occurs, the new membrane permits the cell to expand.

Proteins Involved in Cytokinesis

A large cortical complex called actin, myosin, septins, and other proteins organize these cytoskeletal components into the contractile ring. Many researchers believe that proteins found in the plasma membrane are responsible for the adhesion of the contractile ring to the cell membrane. It is speculated that the contractile ring is similar to other cortical-actin structures in the cell, such as focal adhesions and the accompanying stress fibers.

1. Actinomycin D

• Actin’s role in cytokinesis has been recognized for quite some time and is validated by a number of studies. (1) In virtually all cell types, actin is concentrated in a ring at the site of cell division. (2) Anti-actin medications, such as cytochalasin, impede cytokinesis. (3) Actin mutations in budding yeast generate massive multinucleated cells with cytokinesis and polarity problems. (4) In various species, mutations in genes encoding a range of actin-associated proteins impact cytokinesis.

2. Myosin

• Traditional theories have proposed that cytoplasmic myosin II, the non-muscle form of classical long-tailed myosin, drives cytokinesis.
• Phosphorylation of essential amino acids controls motile activity and multimer formation, making myosin activity a likely candidate for a main control point during cytokinesis.
• Myosin II in the cytoplasm is a hexamer made up of two pairs of polypeptides, the 200 kDa heavy chain (MHC), the 18 kDa essential light chain (ELC), and the 20 kDa regulatory light chain (RLC).

3. Septins

• Genetic information for septins, a class of proteins involved in interactions between the plasma membrane and the cortical cytoskeleton, is present in the genomes of most eukaryotic species (including cytokinesis).
• Most septin proteins have a C-terminal coiled-coil domain, and they all share the characteristic P-loop consensus sequences that are indicative of nucleotide-binding ability.

Additional proteins

Several other cytoskeletal proteins congregate in the vicinity of the cleavage furrow, but their function during cytokinesis is unknown. Proteins that cross-link or bundle actin filaments include a-actinin, filamin, and the recently discovered anillin; actin-membrane linkers include talin and ERM proteins; proteins that regulate F-actin assembly or organization include acidic calponin, which inhibits the ATPase activity of phosphorylated myosin; and transmembrane glycoproteins like CD43 play important roles in cell adhesion and signal transduction. It is important to remember that most of these proteins have not been shown to be contractile ring components using ultrastructural data.

Cytokinesis in Animal Cells

• There is not much difference in the process of cytokinesis between the two types of cell division (mitosis and meiosis). The division plane is established by cellular signals that instruct the cell where to divide.
• The cytokinetic furrow will form around this plane and then pinch off to separate the cells. Abscission is the last step of cytokinesis in animal cells.
• The plasma membranes undergo fission during abscission, and the actin-myosin contractile ring responsible for making the cytokinetic furrow is fully contracted.
• Researchers have yet to pinpoint what triggers cells to choose a certain dividing plane. It’s a complicated procedure that requires a lot of microtubules and cell messages to complete.
• After this location is established, the contractile ring of actin and myosin can be formed. The contraction of muscle cells is caused by the action of the motor proteins actin and myosin.
• Actin filaments abound inside muscle cells, and the protein myosin may bind them together with the help of some ATP. When cells divide in animals, they use the same mechanism.
• At the cell division plane, actin filaments coil into a ring. The myosin proteins subsequently begin to contract, contracting the actin filaments into a tighter ring.
• Finally, the ring was completely devoid of cytoplasm and organelles. The actin-myosin ring and the microtubules that it constricts are all that remain. In order for the cells to split, it is also necessary to divide what is called the midbody structure.
• This takes place as a result of the abscission procedure. There is a disruption in the protein chain and a closing of the plasma membrane. When the glue that separates cells is gone, the cells can finally split apart.
• In some multicellular organisms, cells stay close to one another and even maintain cytoplasmic connections (called gap junctions) with one another. These short connections can originate from fragments of the endoplasmic reticulum that become caught in the midbody, or they can develop independently at a later time.

Cytokinesis in plant cells

• Although plant cells are more stiff than animal cells, they nonetheless undergo a cytokinesis process. The cell wall provides a supplementary protective covering for plants.
• When a plant cell divides, a new extracellular structure must be created to help shape the new cell. For this purpose, plants employ specialized structures called phragmoplasts, which are microtubule spindle organelles.
• Plegmoplasts deliver vesicles containing fresh cell plate material from the old cell wall. Matrixes composed of these elements, such as cellulose, are intricate and durable due to their interactions. When a cell is divided by a plate, the plasma membrane closes and the two halves are no longer attached.
• Similar to the centrosomes in animal cells, the phragmoplast regulates microtubule development and atrophy. The endoplasmic reticulum and the Golgi apparatus are responsible for producing and storing the materials needed to form the new cell plate.
• Next they’re forwarded to the phragmoplast, which constructs the cell plate from the center outward. The image above illustrates this point. The microtubules of the phragmoplast will extend from the center of the cell plate until they reach the plasm membrane.
• After severing this membrane, the cell wall will link to all the cells in its vicinity. Plasmodium sarcoma-associated endoplasmic reticulum (like gap junctions) forms between the cells when endoplasmic reticulum is trapped. The plasmodesmata in plants have been proposed as a means of intercellular communication.

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References

• Srivastava, V., & Robinson, D. N. (2013). Cytokinesis. Encyclopedia of Biological Chemistry, 622–626. doi:10.1016/b978-0-12-378630-2.00499-0
• D’Avino PP, Giansanti MG, Petronczki M. Cytokinesis in animal cells. Cold Spring Harb Perspect Biol. 2015 Feb 13;7(4):a015834. doi: 10.1101/cshperspect.a015834. PMID: 25680833; PMCID: PMC4382743.
• Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Cytokinesis. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26831/
• https://www.biologyonline.com/dictionary/cytokinesis
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• https://biologydictionary.net/cytokinesis/
• https://www.thoughtco.com/cytokinesis-in-a-cell-cycle-373541