Mitosis - Definition, Phases, Significance, Functions

What is Mitosis? – Mitosis Definition

Mitosis is the phase of the cell cycle in which newly synthesised DNA is segregated and two identical daughter cells with the same number and type of chromosomes as the parent nucleus are produced.

Mitosis is an asexual reproductive process that occurs in unicellular organisms. Continue reading to discover what mitosis is and its different stages.
At the cellular level, reproduction is primarily driven by cell division. Except for germ cells, in which the number of chromosomes is halved, the majority of eukaryotic cells divide so that the ploidy or number of chromosomes remains unchanged.

Mitosis is the phase of the cell cycle in which the nucleus is divided into two daughter nuclei containing an equal amount of genetic material. It follows the G2 phase and is followed by cytoplasmic division after nucleus separation.
Mitosis is necessary for cell development and the replacement of worn-out cells. Abnormalities during mitosis have the potential to modify the DNA, leading to hereditary diseases.

Features of Mitosis

In each cycle of cell division, the parent cell divides into two daughter cells.

The cell is also referred to as equational cell division since the number of chromosomes in the parent and daughter cells is same.
Mitosis in plants results in the growth of vegetative parts such as the root tip, stem tip, etc.
In this procedure, segregation and combination do not occur.

The Cell Cycle

Mammalian cells that divide undergo a 24-hour cell cycle during which they grow, reproduce their DNA, and divide into two daughter cells.
The cell cycle consists of interphase, which lasts.22 h, and mitosis, which lasts.2 h.
Interphase is divisible into three sections. DNA is replicated or manufactured during S phase or the synthetic phase of interphase.

First gap (G1) phase is the period between the end of mitosis and the beginning of S phase.
During G1, cells prepare for division by undergoing growth. In the absence of nutrients or growth signals, however, cell cycle progression can be slowed or halted.
Gap 2 refers to the period following DNA replication and preceding the commencement of cell division (G2).

During the G2 phase, cells assess DNA replication; if damaged or unreplicated DNA is discovered, the cell cycle is postponed. In the absence of DNA damage, cells advance through G2 and enter mitosis.

The cell cycle is the series of events that occur between the end of one nuclear division and the commencement of the next. The cell cycle consists of the three cycles listed below.

Chromosome cycle: In the chromosome cycle, DNA synthesis and mitosis alternate (or karyokinesis or nuclear division). During DNA synthesis, each double-helical DNA molecule is reproduced into two identical daughter DNA molecules, which are eventually separated during mitosis.

Cytoplasmic cycle: In the cytoplasmic cycle, cell division alternates with cell growth (or cytoplasmic division). During cell development, several other cell components (RNA, proteins, and membranes) double in number, and during cytokinesis, the entire cell divides in half. Typically, karyokinesis is followed by cytokinesis, but occasionally cytokinesis does not follow karyokinesis, resulting in a multinucleate cell, as in the case of Drosophila egg cleavage.
Centrosome cycle: Both of the preceding cycles require the centrosome to be reliably inherited and precisely duplicated in order to create the two poles of the mitotic spindle; hence, centrosome cycle is the third component of cell cycle.

Howard and Pelc (1953) identified four phases or stages of the cell cycle: G1, S, G2, and M. The classical interphase combines the G1 phase, S phase, and G2 phase.

Spindle Structure

At metaphase, the mitotic spindle is a chromosome-aligned microtubule-based structure with bilateral symmetry.
The spindle is bipolar: the two spindle poles define the spindle axis and are where chromatids are transported during anaphase.
At the spindle poles of numerous animal cells is a centrosome composed of a pair of centrioles and accompanying pericentriolar microtubule-nucleating material.

Microtubules, dynamic cytoskeletal filaments that emanate from the two spindle poles, make up the majority of the spindle’s structural components.
Chromosomes are attached to the spindle by microtubules, which are crucial for chromosome movement. Spindle microtubules are categorised according to their position within the spindle.
Interpolar microtubules emanate from each spindle pole and converge in the spindle’s centre. Kinetochore microtubules have one end imbedded in the kinetochore, a specific attachment point on the chromosome.

Astral microtubules, the third type of spindle microtubules, extend from the centrosomes toward the cell cortex.
Astral microtubules aid in spindle positioning and in determining the position of the contractile ring during cytokinesis.
There is considerable variety in spindle structure between various cell types. In cells of higher plants, the spindle pole is unfocused and devoid of centrioles.

Also anastral, plant spindles lack astral microtubules. At either end of the spindle in yeasts that undergo closed mitosis within an intact nuclear envelope is a spindle pole body.
The spindle pole body nucleates microtubules that radiate into the nucleus and cytoplasm.
Additionally, the quantity of spindle microtubules differs between cell types. Each kinetochore is connected to the spindle pole body by a single microtubule in yeast.

In comparison, each kinetochore in a typical mammal cell contains 20 microtubules, and each half-spindle contains hundreds of microtubules.
Despite these variations in spindle form, mitosis always results in the precise distribution of duplicated genetic material across the two daughter cells.

Mitosis - Definition, Phases, Significance, Functions — Mitosis – Definition, Phases, Significance, Functions

Phases of mitosis

1. Interphase

The interphase is distinguished by the following characteristics:

The nuclear envelope is undamaged.
The chromosomes exist as diffuse, lengthy, coiled, and hardly discernible chromatin fibres.
The amount of DNA doubles. Due to the accumulation of ribosomal RNA (rRNA) and ribosomal proteins in the nucleolus, the latter grows substantially.

In animal cells, a daughter pair of centrioles arises next to the current pair, so an interphase cell contains two pairs of centrioles.
Net membrane biosynthesis increases prior to cell division in animal cells (mitosis).
This excess membrane appears to be stored as blebs on the surface of dividing cells.

1. Prophase

a. Chromosome Condensation

Prior to mitosis, each chromosome is copied during the S phase of interphase, resulting in two identical sister chromatids that are held together by protein complexes known as cohesions.
Sister chromatids are extremely lengthy DNA strands that must be compacted before to division. This process is known as chromosome condensation, and it occurs when condensins coil the DNA into loops.
As prophase advances, the chromosomes become increasingly visible in the nucleus, first as threads or strands and then as highly condensed rod-like structures known as chromosomes, each of which is composed of two chromatids.

The name mitosis is derived from the Greek word mito, which means thread, in reference to the emergence of thread-like structures within the prophase nucleus.

b. Kinetochore Assembly

To precisely separate the chromatids into the two daughter nuclei, they must be linked to the spindle machinery and then moved by it.
The centromere is an indentation or main constriction that distinguishes mitotic chromosomes.

The centromere is a region of frequently repeated DNA sequences that serves as the assembly site for kinetochores, which are plate-like structures.
The kinetochore links each chromatid to the spindle via kinetochore microtubules and also serves as a signalling apparatus that governs cell cycle progression.
To promote the connection of each chromatid with microtubules from different spindle poles, the kinetochores of sister chromatids are organised so that their binding sites for spindle microtubules face in opposite orientations.

c. Centrosome Separation

During S phase of interphase, the centrosome is duplicated in animal cells, and the two daughter centrosomes remain next to one another.
Centrosome duplication guarantees that two centrosomes are present at the start of mitosis to generate the spindle poles and that each daughter cell inherits one centrosome.
During prophase, duplicate centrosomes detach and move to opposing sides of the nucleus to form a bipolar spindle.

During prophase, the capacity of each centrosome to nucleate microtubules increases rapidly.

2. Prometaphase

a. Nuclear Envelope Breakdown

In cells undergoing an open mitosis, the collapse of the nuclear envelope signifies the start of prometaphase.
Microtubules facilitate the disintegration of the nuclear envelope, which fragments the nuclear membrane into minute vesicles and releases the chromosomes into the cytoplasm.

b. Spindle Assembly

Spindle formation requires active microtubules. Because microtubules are polar polymers, the dynamic characteristics of their two ends are distinct.
The less dynamic end, referred to as the minus end, is connected with the spindle pole material, whereas the more dynamic end, referred to as the plus end, extends toward the cell’s periphery.
Microtubule plus ends exhibit a characteristic known as dynamic instability, in which they randomly transition between assembly and disassembly phases.

As a result of dynamic instability, the plus ends of microtubules repeatedly explore the cytoplasm as they expand and shrink.
Mitotic cell microtubules are more dynamic than interphase cell microtubules because they switch from growing to shortening more frequently and continue to shorten before restarting growth.
A population of relatively short, highly active microtubules radiates from each spindle pole as a result. Chromosomes are released into the cytoplasm with rupture of the nuclear membrane, and dynamic microtubules can connect with each kinetochore.

In order to maintain a link between a microtubule and kinetochore, the microtubule becomes less dynamic after the connection is established.
Mitosis is delayed if even a single chromosome is not connected to the mitotic spindle, which is monitored by cells. The name of this regulatory system is spindle assembly checkpoint or metaphase checkpoint.
Spindle assembly is dependent on microtubule motor proteins, which utilise the energy of ATP hydrolysis to do work in cells.

The direction of motion, either toward the minus end or the plus end of the microtubule, is determined by the kind and features of the motor protein.
Multifunctional motors or motor protein complexes can exert force on microtubules via cross-linking and sliding adjacent microtubules, or by binding to one microtubule while walking along an adjacent one.
Alternately, the nonmotor component of the motor protein can connect to a tethered structure, such as the cell cortex, whereas the motor region exerts a pulling force on the microtubule.

At the kinetochores, between interpolar microtubules, along kinetochore microtubules, and at the cell cortex exist motor proteins. During mitosis, motor proteins contribute to diverse movements.
During centrosome separation in prophase, motors situated between antiparallel interpolar microtubules aid in the separation of the two spindle poles. Motors contribute to the early bipolar attachment of chromosomes to spindle microtubules at the kinetochores.
Following initial attachment, motors also contribute to congression, the migration of chromosomes to the spindle equator or metaphase plate.

In the constructed spindle, the length is maintained by a balance of the activities of oppositely directed motor proteins; some motors tend to pull the poles together, while others act to push the poles apart.
In mitotic cells, motor proteins also contribute to spindle placement and the dynamic turnover of microtubules. Spindle formation can also occur without centrosomes.
In this circumstance, microtubules accumulate near chromosomes and are then sorted into a bipolar array by motor proteins. Spindle development is facilitated by active microtubules and motor proteins both in the presence and absence of centrosomes.

3. Metaphase

When all of the duplicated chromosomes are aligned midway between the two spindle poles, also known as the metaphase plate or spindle equator, the cell is in metaphase.
At metaphase, spindle length remains generally constant, while spindle microtubules are dynamic.
Spindle microtubules continually build at their plus ends and deconstruct at their minus ends, accompanied with poleward translocation of the polymer.

During metaphase and anaphase of mitosis, markers placed on spindle microtubules are observed to travel slowly toward the poles due to flux.
In metaphase, not only are microtubules dynamic, but chromosomes oscillate toward and away from each spindle pole.
Additionally, chromosomes are under strain and prepared for subsequent movement during anaphase. If the connection between one sister chromatid and the pole is severed at metaphase, the chromosome shifts to the pole to which it remains attached, illustrating that the metaphase position is the product of an equilibrium between pole-directed forces operating on the chromosomes.

4. Anaphase

Sister chromatids split and migrate to the spindle poles during anaphase.
Anaphase is comprised of two phases, A and B. During anaphase A, chromosomes travel to the spindle poles and kinetochore fibre microtubules shorten; during anaphase B, spindle poles move apart as interpolar microtubules stretch and slide past one another.
Numerous cells undergo both anaphase A and B motions, albeit in some instances one motion predominates. Poleward movement in anaphase requires separation of the paired sister chromatids.

Chromatid separation is a consequence of the proteolytic destruction of components that connect chromatids at the centromere.
The activity of the anaphase-promoting complex, which controls cell cycle progression, initiates degradation. Chromatid separation is observable even in the absence of microtubules, as it is not caused by microtubules and motor proteins.
Although the movement of chromosomes to the spindle poles during anaphase has captivated biologists for decades, the molecular basis for this motion is still contentious and incompletely understood.

As the chromosomes travel poleward during anaphase A, the kinetochore microtubules must shrink. Spindle flux measurements reveal that microtubule subunit loss occurs at the spindle poles during anaphase.
In several cells, however, the rate of chromosomal movement exceeds the rate of subunit loss at the pole, therefore subunit loss must also take place at the kinetochore. Chromosome mobility is caused by the synthesis and disassembly of microtubule polymers, as established by pioneering research on mitosis in embryonic cells that are still alive.
This research led to the hypothesis that the disintegration of microtubules promotes chromosomal movement. The discovery of molecular motors at the kinetochore led to the alternative notion that forces created by molecular motors drive chromosomal mobility.

One idea is that chromosome motion is powered by molecular motors, but the rate of chromosome motion is limited by the breakdown of kinetochore microtubules.
Alternately, chromosomal mobility may be caused by disintegration, and motors may bind the chromosomes to the shortening fibre.
Mitotic integrity is of highest importance, which may be reflected by the presence of possibly redundant pathways for chromosomal mobility.

5. Telophase

During telophase, the chromosomes decondense and the nuclear membrane begins to rebuild, bringing the cell back to its interphase state.
The short, active microtubules of the spindle are replaced by the less dynamic, cell-peripheral microtubules of the interphase array.
In preparation for cytokinesis, the contractile ring assembles midway between the spindle poles during telophase.

Cytokinesis

DNA synthesis and mitosis are linked to cytoplasmic division, or cytokinesis – the separation of cytoplasm into two distinct cells.
During cytokinesis, the cytoplasm divides via the cleavage process. The mitotic spindle plays a crucial function in defining the location and timing of cleavage.
Typically, cytokinesis begins during anaphase and continues through telophase and interphase.

In animal cells, the first indication of cleavage is puckering and furrowing of the plasma membrane during anaphase.
The furrowing always occurs perpendicular to the long axis of the mitotic spindle in the plane of the metaphase plate.
In fertilised sand dollar eggs, a cleavage furrow tends to emerge midway between asters arising from two centrosomes.

Contraction of a ring consisting primarily of actin filaments causes cleavage.
Unidentified attachment proteins bind the contractile ring filament bundle to the cytoplasmic face of the plasma membrane.
Once the contractile ring is completed in early anaphase, it generates sufficient force to bend a thin glass needle put into the cell.

This force is evidently produced by the muscle-like sliding of actin and myosin filaments in the contractile ring. The interaction between actin and myosin pushes the plasma membrane into a furrow.
During a typical cytokinesis, the contractile ring does not increase in thickness as the furrow invaginates, indicating that it continuously loses filaments and shrinks in size.
When cleavage concludes, the contractile ring is eventually eliminated, and the plasma membrane of the cleavage furrow narrows to create the midbody, which serves as a tether between two daughter cells.

The midbody contains the remnants of the two sets of polar microtubules, which are densely packed into a matrix. Cytokinesis significantly increases the overall cell surface area when two cells are created from a single cell.
Therefore, the two daughter cells that arise from cytokinesis require a greater amount of plasma membrane than the parent cell.
In M phase, prior to cytokinesis, big membrane-bounded organelles such as the Golgi apparatus and endoplasmic reticulum fragment into smaller fragments and vesicles; this may ensure their uniform distribution into daughter cells during cytokinesis.

Physiology of Cell Cycle and Mitosis

The following components of the cell cycle and mitosis require elaboration:

1. Regulation of mitotic chromosome cycle

Mitotic chromosomal cycles are governed by the three regulatory factors (i.e., diffusible proteins) listed below: The S-phase activator, which ordinarily occurs in the cytoplasm only during S-phase and “activates” DNA synthesis (Rao and Johanson, 1970). 2. The M-phase promoting factor (MPF) that is exclusive to the cytoplasm of cells in M-phase and causes chromosomal condensation. DNA-dependent M-phase delaying factor present in S-phase cytoplasm and inhibiting the pathway leading to MPF synthesis.
In the cell cycle, the sudden arrival and disappearance of these diffusible components in the cytoplasm are landmark events.

The causal linkages between these components ensure that the events of the chromosomal cycle will always occur in a predetermined sequence, hence preventing catastrophic mishaps such as chromosome condensation during DNA synthesis.
Each step depends on the one that preceded it (i.e. all processes of chromosome cycle are linked together as dependent sequence).
Thus, (1) the cell cannot pass through mitosis until MPF has been produced; (2) MPF cannot be produced until the M-phase-delaying-factor has disappeared; (3) the M-phase-delaying-factor and S-phase activator cannot disappear until DNA-synthesis has ended; (4) DNA synthesis cannot end until all of the DNA has replicated; (5) the DNA cannot begin to replicate until the DNA rereplication block has been removed by the cell’s passage through mitosis into G1;

MPF is a big protein composed of two subunits: an inactive subunit and a kinase subunit that can phosphorylate (and activate) the inactive subunit and other molecules (called self-activation).
Thus, MPF kinase directly phosphorylates various substances, including histone H1, encouraging chromosomal condensation; and it may be through a cascade of phosphorylation that MPF causes all the complicated events of mitosis, including nuclear envelope collapse and cytoskeletal alteration (e.g., formation of mitotic spindle).

2. Dissolution and formation of nuclear envelope during mitosis

At least three components of the nuclear envelope complex must be taken into account during its disassembly (during prophase) and reassembly (during telophase): 1. the outer and inner nuclear membranes; 2. the nuclear lamina composed of lamin proteins; and 3. the nuclear pores.

During prophase, MPF phosphorylates several proteins, and phosphorylation of nuclear lamins regulates the disintegration and repair of the nuclear envelope.
Lamins are phosphorylated at several places along each polypeptide chain, resulting in their disassembly and disruption of the nuclear lamina.
Afterwards, maybe in response to a separate signal, the nuclear envelope proper disassembles into tiny membrane vesicles.

In less than an hour (from prophase to prometaphase), Maul (1977) reports that nearly all 4000 holes vanish from the nuclear membranes of cultivated mammalian cells.
During mitosis, these pore complexes have been discovered on chromosomes. It is believed that the abrupt transition from metaphase to anaphase initiates the dephosphorylation of numerous prophase-phosphorylated proteins, including histone H1 and the lamins.
Shortly later, during telophase, nuclear membrane vesicles associated with the surface of individual chromosomes are fused to re-form the nuclear membranes that partially encase clusters of chromosomes prior to re-forming the full nuclear envelope.

During this step, the presynthesized nuclear pores reassembly and the dephosphorylated lamins reassociate to form the nuclear lamina; one of the lamina proteins (lamin B) remains linked with nuclear membrane fragments during mitosis and may aid in nuclear assembly.

3. Role of cytoskeleton in mitosis

It has been argued that the chromosomes in mitosis are analogous to the body at a funeral, in that they give the rationale for the events but do not actively participate in them.
Their unique cytoskeletal structures that arise briefly during M phase play an active role.

Mitotic daughter chromosomes migrate poleward along a bipolar mitotic spindle formed of microtubules and their accompanying proteins, which forms initially.
The second cytoskeletal structure required in animal cells during the M phase is a contractile ring of microfilaments and myosin that arises slightly later immediately under the plasma membrane; it is intended for cytokinesis.
The third component of the cytoskeleton is a meshwork of intermediate filaments that surrounds the interphase nucleus, elongates during mitosis to contain the two daughter nuclei, and then separates in two along the cleavage furrow.

Working of mitotic spindle during anaphase

During anaphase A, a chromosome travels from the metaphase plate to the spindle pole under the influence of a force that is remarkably strong.
Hydrodynamic analysis has determined that to move a chromosome, a force of approximately 10-11 dynes is required, and that the entire movement, from the chromosome’s equator to its pole, may need approximately 30 ATP molecules.
As each chromosome advances poleward, its kinetochore microtubules breakdown until, by telophase, they have completely vanished.

By injecting tagged tubulin into cells during metaphase, the site of subunit loss can be identified.
The labelled subunits are added to the kinetochore end of kinetochore microtubules and then removed as anaphase A progresses, demonstrating that the kinetochore “eats” its way poleward along its microtubules during anaphase A. Nonetheless, microtubule disassembly at kinetochores, poles, or both locations is likely required for equator-to-pole migration.
In addition, the process by which chromosomal kinetochore ascends the spindle during anaphase A remains unknown. Nevertheless, the following three models shed some light on the matter:
- The kinetochore degrades ATP to travel along its associated microtubule, with the microtubule’s plus end depolymerizing as it gets exposed.
- The depolymerization of the microtubule itself causes the kinetochore to passively migrate in order to maximise its binding energy to the microtubule.
- A network of elastic protein filaments may connect the kinetochore to the pole and pull it steadily in that direction. In this instance, microtubules only regulate chromosomal mobility.

In mammalian cells, anaphase B begins shortly after the chromatids begin their journey to the poles and concludes when the spindle is around 1.5 to 2.0 times the length of metaphase (15 times increase in certain protozoa).
In contrast to anaphase A, anaphase B is accompanied by polymerization of the plus ends of polar microtubules.
At addition, the polar microtubules of each half-spindle overlap in a central region close to the spindle’s equator (e.g., diatoms).

In the region of overlap, these two sets of antiparallel polar microtubules appear to separate during anaphase B. A force-generating protein similar to Dynein may be involved in such guided chromosomal movement.

Significance of Mitosis

This is the significance of mitosis for living organisms:

Mitosis helps the cell maintain a healthy size.

It contributes to the preservation of a balance between DNA and RNA in the cell.
Mitosis gives the chance for the growth and development of an organism’s organs and body.
Mitosis is used to replace aged, ageing, and decomposing cells in the body.

Mitosis plays a role in the asexual reproduction of certain species.
Mitosis is necessary for the gonads and sex cells to proliferate in quantity.
Mitosis is involved in both the cleavage of the egg during embryogenesis and the division of the blastema during blastogenesis.

Functions of Mitosis

The two most important functions of mitosis are as follows:

Mitosis contributes to the growth of an organism. Mitosis in unicellular organisms is the process of asexual reproduction.
Mitosis aids in the regeneration of injured tissues. When the cells adjacent to the injured cells cannot detect their neighbours, they initiate mitosis. The dividing cells reach the injured cells and cover them.

References

Sullivan, K. F. (2001). Mitosis. Encyclopedia of Genetics, 1224–1227. doi:10.1006/rwgn.2001.0839
Teusel, F., Henschke, L., & Mayer, T. U. (2018). Small molecule tools in mitosis research. Methods in Cell Biology, 137–155. doi:10.1016/bs.mcb.2018.03.005
Wadsworth, P., & Rusan, N. M. (2004). Mitosis. Encyclopedia of Biological Chemistry, 743–747. doi:10.1016/b0-12-443710-9/00094-6
Schatten, H. (2013). Mitosis. Brenner’s Encyclopedia of Genetics, 448–451. doi:10.1016/b978-0-12-374984-0.00962-1