Overview ''
embryo The primary result of mitosis and cytokinesis is the transfer of a parent cell's
genome into two daughter cells. The genome is composed of a number of chromosomes—complexes of tightly coiled
DNA that contain
genetic information vital for proper cell function. Because each resultant daughter cell should be
genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurs during the
S phase of interphase. Cells may also temporarily or permanently leave the cell cycle and enter
G0 phase to stop dividing. This can occur when cells become overcrowded (
density-dependent inhibition) or when they
differentiate to carry out specific functions for the organism, as is the case for
human heart muscle cells and
neurons. Some G0 cells have the ability to re-enter the cell cycle. DNA double-strand breaks can be
repaired during interphase by two principal processes. The first process,
non-homologous end joining (NHEJ), can join the two broken ends of DNA in the
G1,
S and
G2 phases of interphase. The second process,
homologous recombinational repair (HRR), is more accurate than NHEJ in repairing double-strand breaks. HRR is active during the S and G2 phases of interphase when
DNA replication is either partially accomplished or after it is completed, since HRR requires two adjacent
homologs. Interphase helps prepare the cell for mitotic division. It dictates whether the mitotic cell division will occur. It carefully stops the cell from proceeding whenever the cell's DNA is damaged or has not completed an important phase. The interphase is very important as it will determine if mitosis completes successfully. It will reduce the amount of damaged cells produced and the production of cancerous cells. A miscalculation by the key Interphase proteins could be crucial as the latter could potentially create cancerous cells.
Mitosis s of
chromatids
Preprophase (plant cells) In plant cells only, prophase is preceded by a
preprophase stage. In highly
vacuolated plant cells, the nucleus has to migrate into the center of the cell before mitosis can begin. This is achieved through the formation of a
phragmosome, a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division. In addition to phragmosome formation, preprophase is characterized by the formation of a ring of microtubules and
actin filaments (called
preprophase band) underneath the plasma membrane around the equatorial plane of the future mitotic
spindle. This band marks the position where the cell will eventually divide. The cells of higher plants (such as the
flowering plants) lack
centrioles; instead, microtubules form a spindle on the surface of the nucleus and are then organized into a spindle by the chromosomes themselves, after the nuclear envelope breaks down.
Prophase during mitosis During prophase, which occurs after G2 interphase, the cell prepares to divide by tightly condensing its chromosomes and initiating mitotic spindle formation. During interphase, the genetic material in the nucleus consists of loosely packed
chromatin. At the onset of prophase, chromatin fibers condense into discrete chromosomes that are typically visible at high magnification through a
light microscope. In this stage, chromosomes are long, thin, and thread-like. Each chromosome has two chromatids. The two chromatids are joined at the centromere.
Gene transcription ceases during prophase and does not resume until late anaphase to early G1 phase. The
nucleolus also disappears during early prophase. Close to the nucleus of an animal cell are structures called
centrosomes, consisting of a pair of
centrioles surrounded by a
loose collection of proteins. The centrosome is the coordinating center for the cell's
microtubules. A cell inherits a single centrosome at cell division, which is
duplicated by the cell before a new round of mitosis begins, giving a pair of centrosomes. The two centrosomes polymerize
tubulin to help form a
microtubule spindle apparatus.
Motor proteins then push the centrosomes along these microtubules to opposite sides of the cell. Although centrosomes help organize microtubule assembly, they are not essential for the formation of the spindle apparatus, since they are absent from plants,
Prometaphase At the beginning of prometaphase in animal cells, phosphorylation of
nuclear lamins causes the
nuclear envelope to disintegrate into small membrane
vesicles. As this happens, microtubules invade the nuclear space. This is called
open mitosis, and it occurs in some multicellular organisms. Fungi and some
protists, such as
algae or
trichomonads, undergo a variation called
closed mitosis where the spindle forms inside the nucleus, or the microtubules penetrate the intact nuclear envelope. A number of
polar microtubules find and interact with corresponding polar microtubules from the opposite centrosome to form the mitotic spindle. In most animal cells, anaphase A precedes anaphase B, but some vertebrate egg cells demonstrate the opposite order of events.
Telophase during mitosis Telophase (from the
Greek word
τελος meaning "end") is a reversal of prophase and prometaphase events. At telophase, the polar microtubules continue to lengthen, elongating the cell even more. If the nuclear envelope has broken down, a new nuclear envelope forms using the membrane vesicles of the parent cell's old nuclear envelope. The new envelope forms around each set of separated daughter chromosomes (though the membrane does not enclose the centrosomes) and the nucleolus reappears. Both sets of chromosomes, now surrounded by new nuclear membrane, begin to "relax" or decondense. Mitosis is complete. Each daughter nucleus has an identical set of chromosomes. Cell division may or may not occur at this time depending on the organism.
Cytokinesis undergoing
cytokinesis, with the
cleavage furrow being clearly visible
Cytokinesis is not a phase of mitosis, but rather a separate process necessary for completing cell division. In animal cells, a
cleavage furrow (pinch) containing a
contractile ring, develops where the metaphase plate used to be, pinching off the separated nuclei. In both animal and plant cells, cell division is also driven by vesicles derived from the
Golgi apparatus, which move along microtubules to the middle of the cell. In plants, this structure coalesces into a cell plate at the center of the
phragmoplast and develops into a cell wall, separating the two nuclei. The phragmoplast is a microtubule structure typical for higher plants, whereas some green algae use a
phycoplast microtubule array during cytokinesis. Each daughter cell has a complete copy of the genome of its parent cell. The end of cytokinesis marks the end of the M-phase. There are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei. The most notable occurrence of this is among the
fungi,
slime molds, and coenocytic algae, but the phenomenon is found in various other organisms. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of
fruit fly embryonic development. == Function ==