Cytokinesis is the process during cell division in which the cytoplasm of a
single eukaryotic cell is divided to form two daughter cells. It usually
initiates during the early stages of mitosis, and sometimes meiosis,
splitting a mitotic cell in two, to ensure that chromosome number is
maintained from one generation to the next. After cytokinesis two cells will
be formed that are exact copies of the original cell. After cytokinesis, each
daughter cell is in the interphase portion of the cell cycle. In animal
cells, one notable exception to the normal process of cytokinesis is
oogenesis, where the ovum takes almost all the cytoplasm and organelles,
leaving very little for the resulting polar bodies, which then die. Another
form of mitosis without cytokinesis occurs in the liver, yielding
multinucleate cells. In plant cells, a dividing structure known as the cell
plate forms within 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. Animal cell cytokinesis
= Contractile ring positioning = During different, proliferative
divisions, animal cell cytokinesis begins shortly after the onset of sister
chromatid separation in the anaphase of mitosis. A contractile ring, made of
non-muscle myosin II and actin filaments, assembles equatorially at the
cell cortex. Myosin II uses the free energy released when ATP is hydrolysed
to move along these actin filaments, constricting the cell membrane to form a
cleavage furrow. Continued hydrolysis causes this cleavage furrow to ingress,
a striking process that is clearly visible through a light microscope.
Ingression continues until a so-called midbody structure is formed and the
process of abscission then physically cleaves this midbody into two.
Abscission depends on septin 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.
The position at which the contractile ring assembles is dictated by the
mitotic spindle. This seems to depend upon the GTPase RhoA, which influences
several downstream effectors to promote myosin activation) and actin filament
assembly at a particular region of the cell cortex.
Simultaneous with contractile ring assembly during prophase, a microtubule
based structure termed the central spindle forms when non-kinetochore
microtubule fibres are bundled between the spindle poles. A number of different
species including H. sapiens, D. melanogaster and C. elegans require the
central spindle in order to efficiently undergo cytokinesis, although the
specific phenotype described when it is absent varies from one species to the
next. Seemingly vital for the formation of the central spindle is a
heterotetrameric protein complex called centralspindlin. Along with associated
factors, 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 chromatids separate during the anaphase portion of 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. Cytokinesis
happens only after APC binds with CDC20. This allows for the separation of
chromosomes and myosin to work simultaneously.
Plant cell cytokinesis Due to the presence of a cell wall,
cytokinesis in plant cells is significantly different from that in
animal cells, Rather than forming a contractile ring, plant cells construct
a cell plate in the middle of the cell. The stages of cell plate formation
include creation of the phragmoplast, an array of microtubules that guides and
supports the formation of the cell plate; trafficking of vesicles to the
division plane and their fusion to generate a tubular-vesicular network;
continued fusion of membrane tubules and their transformation into membrane
sheets upon the deposition of callose, followed by deposition of cellulose and
other cell wall components; recycling of excess membrane and other material from
the cell plate; and fusion with the parental cell wall
The phragmoplast is assembled from the remnants of the mitotic spindle, and
serves as a track for the trafficking of vesicles 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 as the source of these vesicles, but other
studies have suggested that they contain endocytosed material as well.
These tubules then widen and fuse laterally with each other, eventually
forming a planar, fenestrated sheet [8]. As the cell plate matures, large amounts
of membrane material are removed via clathrin-mediated endocytosis [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 passing through them, and are
thought to be the precursors of plasmodesmata [8].
The construction of the new cell wall begins within the lumen of the narrow
tubules of the young 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, hemicelluloses, and
arabinogalactan proteins carried by the secretory vesicles that fuse to form the
cell plate. The next component to be added is 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, the primary component of a mature cell wall [6]. The
middle lamella develops from the cell plate, serving to bind the cell walls of
adjoining cells together. Etymology and pronunciation
The word cytokinesis uses combining forms of cyto- + kine- + -sis, New Latin
from Classical Latin and Ancient Greek, reflecting "cell" and kinesis.
See also Diploid
References Further reading
The Molecular Requirements for Cytokinesis by M. Glotzer, Science 307,
1735 "Animal Cytokinesis: from parts list to
mechanism" by Eggert, U.S., Mitchison, T.J., Field, C.M., Annual Review of Cell
Biology 75, 543-66 Campbell Biology, 580-582
More description and nice images of cell division in plants, with a focus on
fluorescence microscopy Nanninga, Nanne. Cytokinesis in
Prokaryotes and Eukaryotes: Common Principles and Different Solutions