Cytokinesis - AbsoluteAstronomy.com

Cytokinesis is the process in which the cytoplasm

Cytoplasm

The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis

Mitosis

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

, and sometimes meiosis

Meiosis

Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

, splitting a binucleate cell in two, to ensure that chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

number is maintained from one generation to the next. In animal cells, one notable exception to the normal process of cytokinesis is oogenesis

Oogenesis

Oogenesis, ovogenesis or oögenesis is the creation of an ovum . It is the female form of gametogenesis. The male equivalent is spermatogenesis...

(the creation of an ovum

Ovum

An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

in the ovarian follicle

Ovarian follicle

Ovarian follicles are the basic units of female reproductive biology, each of which is composed of roughly spherical aggregations of cells found in the ovary. They contain a single oocyte . These structures are periodically initiated to grow and develop, culminating in ovulation of usually a single...

of the ovary

Ovary

The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...

), where the ovum takes almost all the cytoplasm and organelles, leaving very little for the resulting polar bodies, which then die. In plant cells, a dividing structure known as the cell plate forms across the centre of the cytoplasm and a new cell wall forms between the two daughter cells.

Cytokinesis is distinguished from the prokaryotic process of binary fission.

Contractile ring positioning

During different proliferative divisions,barnacles and animal cell cytokinesis begins shortly after the onset of sister chromatid

Chromatid

A chromatid is one of the two identical copies of DNA making up a duplicated chromosome, which are joined at their centromeres, for the process of cell division . They are called sister chromatids so long as they are joined by the centromeres...

separation in the anaphase

Anaphase

Anaphase, from the ancient Greek ἀνά and φάσις , is the stage of mitosis or meiosis when chromosomes move to opposite poles of the cell....

of mitosis

Mitosis

. A contractile ring, made of non-muscle myosin

Myosin

Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...

II and actin

Actin

Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...

filaments, assembles equatorially (in the middle of the cell) at the cell cortex

Cell cortex

The cell cortex is a specialized layer of cytoplasm on the inner face of the plasma membrane that functions as a mechanical support of the plasma membrane. In animal cells it is an actin-rich layer responsible for movements of the cell surface. In plant cells, the cell cortex is reinforced by...

(adjacent to the cell membrane). Myosin II uses the free energy released when ATP

Adenosine triphosphate

Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

is hydrolysed to move along these actin filaments, constricting the cell membrane to form a cleavage furrow

Cleavage furrow

In cell biology, the cleavage furrow is the indentation that begins the process of cleavage, by which animal and some algal cells undergo cytokinesis. The same proteins responsible for muscle contraction, actin and myosin begin the process of forming the cleavage furrow. This can only happen in...

. Continued hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

causes this cleavage furrow to ingress (move inwards), a striking process that is clearly visible through a light microscope. Ingression continues until a so-called midbody structure

Midbody (cell biology)

The midbody is a transient structure found in mammalian cells and is present near the end of cytokinesis just prior to the complete separation of the dividing cells...

(composed of electron-dense, proteinaceous material) is formed and the process of abscission

Abscission

Abscission is a term used in several areas of biology. In plant sciences it most commonly refers to the process by which a plant drops one or more of its parts, such as a leaf, fruit, flower or seed...

then physically cleaves this midbody into two. Abscission depends on septin

Septins

Septins are evolutionary conserved proteins with essential functions in cytokinesis, and more subtle roles throughout the cell cycle. Much of our knowledge about septins originates from studies of budding yeast Saccharomyces cerevisiae, where they form a ring-like protein scaffold at the mother-bud...

filaments beneath the cleavage furrow, which provide a structural basis to ensure the completion of cytokinesis. After cytokinesis, non-kinetochore microtubules reorganize and disappear into a new cytoskeleton as the cell cycle returns to interphase

Interphase

Interphase is the phase of the cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for cell division. In preparation for cell division, it increases its size and makes a copy of its DNA...

(see also cell cycle

Cell cycle

The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...

).

The position at which the contractile ring assembles is dictated by the mitotic spindle

Mitotic spindle

In cell biology, the spindle fibers are the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...

. This seems to depend upon the GTPase

GTPase

GTPases are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate . The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.-Functions:...

RhoA

RHOA

Ras homolog gene family, member A is a small GTPase protein known to regulate the actin cytoskeleton in the formation of stress fibers. In humans, it is encoded by the gene RHOA....

, which influences several downstream effectors (such as the protein kinases ROCK and citron) to promote myosin

Myosin

activation (by influencing the phosphorylation of Myosin regulatory light chain (rMLC)) and actin

Actin

filament assembly (by regulating formin protein) at a particular region of the cell cortex.

Simultaneous with contractile ring assembly during prophase, a microtubule based structure termed the central spindle (or spindle midzone) forms when non-kinetochore microtubule fibres are bundled between the spindle poles. A number of different species including H. sapiens, D. melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

and C. elegans
Caenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...

require the central spindle in order to efficiently undergo cytokinesis, although the specific phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

described when it is absent varies from one species to the next (for example, certain Drosophila cell types are incapable of forming a cleavage furrow without the central spindle, whereas in both C. elegans embryos and human tissue culture

Tissue culture

Tissue culture is the growth of tissues or cells separate from the organism. This is typically facilitated via use of a liquid, semi-solid, or solid growth medium, such as broth or agar...

cells a cleavage furrow is observed to form and ingress, but then regress before cytokinesis is complete). Seemingly vital for the formation of the central spindle (and therefore efficient cytokinesis) is a heterotetrameric protein complex called centralspindlin. Along with associated factors (such as SPD-1 in C. elegans), centralspindlin plays a role in bundling microtubules to form the spindle midzone during anaphase.

Timing cytokinesis

Cytokinesis must be temporally controlled to ensure that it occurs only after sister anaphase

Anaphase

Anaphase, from the ancient Greek ἀνά and φάσις , is the stage of mitosis or meiosis when chromosomes move to opposite poles of the cell....

separation during normal proliferative cell divisions. To achieve this, many components of the cytokinesis machinery are highly regulated to ensure that they are able to perform a particular function at only a particular stage of the cell cycle

Cell cycle

Plant cell cytokinesis

Due to the presence of a cell wall

Cell wall

The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

, cytokinesis in plant cells is significantly different from that in animal cells. Rather than forming a contractile ring, plant cells construct a cell plate

Cell plate

thumb|300px|Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Toawards the right: Phragmoplast enlarges in a donut-shape towards the outside of the cell, leaving behind mature cell...

in the middle of the cell. The stages of cell plate

Cell plate

formation include (1) creation of the phragmoplast

Phragmoplast

thumb|300px|Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut-shape towards the outside of the cell, leaving behind mature cell...

, an array of microtubules that guides and supports the formation of the cell plate

Cell plate

; (2) trafficking of vesicles to the division plane and their fusion to generate a tubular-vesicular network; (3) continued fusion of membrane tubules and their transformation into membrane sheets upon the deposition of callose

Callose

Callose is a plant polysaccharide. It is composed of glucose residues linked together through β-1,3-linkages, and is termed a β-glucan. It is thought to be manufactured at the cell wall by callose synthases and is degraded by β-1,3-glucanases. It is laid down at plasmodesmata, at the cell...

, followed by deposition of cellulose

Cellulose

Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

and other cell wall

Cell wall

components; (4) recycling of excess membrane and other material from the cell plate

Cell plate

; and (5) fusion with the parental cell wall

Cell wall

The phragmoplast

Phragmoplast

thumb|300px|Phragmoplast and cell plate formation in a plant cell during cytokinesis. Left side: Phragmoplast forms and cell plate starts to assemble in the center of the cell. Towards the right: Phragmoplast enlarges in a donut-shape towards the outside of the cell, leaving behind mature cell...

is assembled from the remnants of the mitotic spindle

Mitotic spindle

In cell biology, the spindle fibers are the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...

, and serves as a track for the trafficking of vesicles

Vesicle (biology)

A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...

to the phragmoplast midzone. These vesicles contain lipids, proteins and carbohydrates needed for the formation of a new cell boundary. Electron tomographic studies have identified the Golgi apparatus

Golgi apparatus

The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....

as the source of these vesicles, but other studies have suggested that they contain endocytosed material as well.

The initial vesicle fusion

Vesicle fusion

Vesicle fusion is the merging of a vesicle with other vesicles or a part of a cell membrane. In the latter case, it is the end stage of secretion from secretory vesicles, where their contents are expelled from the cell through exocytosis at the porosome...

events give rise to dumbbell-shaped membrane structures which have been proposed to grow by additional fusions into a tubular network. These tubules then widen and fuse laterally with each other, eventually forming a planar, fenestrated sheet ^[8]. As the cell plate

Cell plate

matures, large amounts of membrane material are removed via clathrin-mediated endocytosis

Endocytosis

Endocytosis is a process by which cells absorb molecules by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane...

^[7] Eventually, the edges of the cell plate fuse with the parental plasma membrane, often in an asymmetrical fashion, thus completing cytokinesis. The remaining fenestrae contain strands of endoplasmic reticulum

Endoplasmic reticulum

The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...

passing through them, and are thought to be the precursors of plasmodesmata

Plasmodesmata

Plasmodesmata are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them. Species that have plasmodesmata include members of the Charophyceae, Charales and Coleochaetales , as well as all embryophytes, better known...

^[8].

The construction of the new cell wall

Cell wall

begins within the lumen of the narrow tubules of the young cell plate

Cell plate

. The order in which different cell wall components are deposited has been determined largely by immuno-electron microscopy. The first components to arrive are pectins, hemicellulose

Hemicellulose

A hemicellulose is any of several heteropolymers , such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...

s, and arabinogalactan protein

Arabinogalactan protein

Arabinogalactan protein is a type of protein found in plant cell walls. It is heavily glycosylated, with only 2-10% protein. It is acidic and contains hydroxyproline, serine, alanine and glycine amino acids predominantly...

s carried by the secretory vesicles that fuse to form the cell plate. The next component to be added is callose

Callose

, which is polymerized directly at the cell plate by callose synthases. As the cell plate continues to mature and fuses with the parental plasma membrane, the callose is slowly replaced with cellulose

Cellulose

Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

, the primary component of a mature cell walls ^[6].

Bacterial cell cytokinesis

In bacterial cells, a tubulin-like protein called FtsZ was observed to be distributed equally in the cell, but seen to be forming a ring when cytokinesis takes place. The FtsZ ring becomes narrower by GTP hydrolysis. FtsZ recruits other Fts proteins to the site, among other mureine transpeptidases. It is strongly suggested that the polar regions of a bacterium exclude FtsZ, thereby assuring that the contractile ring forms in the middle of the cell.

Contractile ring positioning

Timing cytokinesis

Plant cell cytokinesis

Bacterial cell cytokinesis

Further reading