MITOSIS: PHASES AND THEIR CHARACTERISTICS, FUNCTIONS, AND ORGANISMS - BIOLOGY - 2024

The mitosis is a cell division process, where a cell produces genetically identical daughter cells; two "daughters" with the same chromosomal load are generated for each cell. This division takes place in the somatic cells of eukaryotic organisms.

This process is one of the stages of the cell cycle of eukaryotic organisms, which is comprised of 4 phases: S (DNA synthesis), M (cell division), G1 and G2 (intermediate phases where mRNAs and proteins are produced). Together, the G1, G2, and S phases are considered as an interface. Nuclear and cytoplasmic division (mitosis and cytokinesis) make up the last stage of the cell cycle.

Overview of mitosis. Source: Viswaprabha

At the molecular level, mitosis is initiated by the activation of a kinase (protein) called MPF (Maturation Promoting Factor) and the consequent phosphorylation of a significant number of the cell's component proteins. The latter allows the cell to present the morphological changes necessary to carry out the division process.

Mitosis is an asexual process, since the progenitor cell and its daughters have exactly the same genetic information. These cells are known as diploid because they carry the complete chromosomal load (2n).

Meiosis, on the other hand, is the process of cell division that leads to sexual reproduction. In this process, a diploid stem cell replicates its chromosomes and then divides twice in a row (without replicating its genetic information). Finally, 4 daughter cells are generated with only half the chromosomal load, which are called haploid (n).

Overview of mitosis

Mitosis in single-celled organisms generally produces daughter cells that are very similar to their progenitors. In contrast, during the development of multicellular beings, this process can give rise to two cells with some different characteristics (despite being genetically identical).

This cell differentiation gives rise to the different cell types that make up multicellular organisms.

During the life of an organism, the cell cycle occurs continuously, constantly forming new cells that, in turn, grow and prepare to divide through mitosis.

Cell growth and division are regulated by mechanisms, such as apoptosis (programmed cell death), which allow maintaining a balance, avoiding excess tissue growth. In this way, it is ensured that the defective cells are replaced by new cells, according to the requirements and needs of the body.

How relevant is this process?

The ability to reproduce is one of the most important characteristics of all organisms (from unicellular to multicellular) and of the cells that compose it. This quality ensures the continuity of your genetic information.

Understanding the processes of mitosis and meiosis have played a fundamental role in understanding the intriguing cellular characteristics of organisms. For example, the property of keeping the number of chromosomes constant from one cell to another within an individual, and between individuals of the same species.

When we suffer some type of cut or wound on our skin, we observe how in a matter of days the damaged skin recovers. This occurs thanks to the process of mitosis.

Phases and their characteristics

In general, mitosis follows the same sequence of processes (phases) in all eukaryotic cells. In these phases many morphological changes occur in the cell. Among them the condensation of chromosomes, rupture of the nuclear membrane, separation of the cell from the extracellular matrix and other cells, and the division of the cytoplasm.

In some cases, nuclear division and cytoplasmic division are considered as distinct phases (mitosis and cytokinesis, respectively).

For a better study and understanding of the process, six (6) phases have been designated, called: prophase, prometaphase, metaphase, anaphase and telophase, then cytokinesis is considered as a sixth phase, which begins to develop during anaphase.

Telophase is the last phase of mitosis. Taken from https://commons.wikimedia.org/wiki/File:Mitosepanel.jpg. Via Wikimedia Commons

These phases have been studied since the 19th century through the light microscope, so today they are easily recognizable according to the morphological characteristics that the cell presents, such as chromosomal condensation, and the formation of the mitotic spindle.

Prophase

Prophase. Leomonaci98, from Wikimedia Commons

Prophase is the first visible manifestation of cell division. In this phase, the appearance of chromosomes can be seen as distinguishable forms, due to the progressive compaction of chromatin. This condensation of chromosomes begins with phosphorylation of Histone H1 molecules by MPF kinase.

The condensation process consists of the contraction and therefore the reduction of the magnitude of the chromosomes. This occurs due to the coiling of the chromatin fibers, producing more easily displaceable structures (mitotic chromosomes).

Chromosomes previously duplicated during the S period of the cell cycle, acquire a double-stranded appearance, called sister chromatids, these strands are held together through a region called the centromere. In this phase the nucleoli also disappear.

Mitotic spindle formation

By Silvia3, from Wikimedia Commons

During prophase, the mitotic spindle is formed, made up of microtubules and proteins that make up a set of fibers.

As the spindle forms, the microtubules of the cytoskeleton are disassembled (by deactivating the proteins that maintain their structure), providing the necessary material for the formation of said mitotic spindle.

The centrosome (a membraneless organelle, functional in the cell cycle), duplicated at the interface, acts as the assembly unit of the spindle microtubules. In animal cells, the centrosome has in the center, a pair of centrioles; but these are absent in most plant cells.

The duplicated centrosomes begin to separate from each other while the spindle microtubules assemble in each one of them, beginning to migrate towards opposite ends of the cell.

At the end of prophase, the rupture of the nuclear envelope begins, occurring in separate processes: the disassembly of the nuclear pore, the nuclear lamina and the nuclear membranes. This break allows the mitotic spindle and chromosomes to begin to interact.

Prometaphase

Leomonaci98

At this stage, the nuclear envelope has been completely fragmented, so that the microtubules of the spindle invade this area, interacting with the chromosomes. The two centrosomes have separated, each locating at the poles of the mitotic spindle, at opposite ends of the cells.

Now, the mitotic spindle comprises the microtubules (which extend from each centrosome towards the center of the cell), the centrosomes, and a pair of asters (structures with a radial distribution of short microtubules, which unfold from each centrosome).

The chromatids each developed a specialized protein structure, called the kinetochore, located in the centromere. These kinetochores are located in opposite directions and some microtubules, called kinetochore microtubules, adhere to them.

These microtubules attached to the kinetochore, begin to move to the chromosome from the end of which they extend; some from one pole and others from the opposite pole. This creates a "pull and shrink" effect that, when stabilized, allows the chromosome to end up located between the ends of the cell.

Metaphase

Chromosomes aligned in the equatorial plate of the cell during mitotic metaphase

In metaphase, centrosomes are located at opposite ends of cells. The spindle shows a clear structure, in the center of which the chromosomes are located. The centromeres of these chromosomes are attached to the fibers and aligned in an imaginary plane called the metaphase plate.

The chromatid kinetochores remain attached to the kinetochore microtubules. Microtubules that do not adhere to the kinetochores and extend from opposite poles of the spindle now interact with each other. At this point the microtubules from the asters are in contact with the plasma membrane.

This growth and interaction of microtubules completes the structure of the mitotic spindle, giving it a "bird cage" appearance.

Morphologically, this phase is the one with the least changes, which is why it was considered a resting phase. However, although they are not easily noticeable, many important processes occur in it, in addition to being the longest stage of mitosis.

Anaphase

Source: Leomonaci98, from Wikimedia Commons

During anaphase, each pair of chromatids begins to separate (due to the inactivation of the proteins that hold them together). Separated chromosomes move to opposite ends of the cell.

This migration movement is due to the shortening of the microtubules of the kinetochore, generating a "pull" effect that causes each chromosome to move from its centromere. Depending on the location of the centromere on the chromosome, it may take on a particular shape such as V or J.

Microtubules not adhered to the kinetochore, grow and lengthen by adhesion of tubulin (protein) and by the action of motor proteins that move over them, allowing contact between them to stop. As they move away from each other, the poles of the spindle do as well, lengthening the cell.

At the end of this phase, the groups of chromosomes are located at opposite ends of the mitotic spindle, leaving each end of the cell with a complete and equivalent set of chromosomes.

Telophase

Telophase. Leomonaci98

The telophase is the last phase of nuclear division. The microtubules of the kinetochore disintegrate while the polar microtubules are further elongated.

The nuclear membrane begins to form around each set of chromosomes, using the nuclear envelopes of the progenitor cell, which were like vesicles in the cytoplasm.

In this stage, the chromosomes that are at the cell poles are completely decondensed due to the dephosphorylation of the histone molecules (H1). The formation of the elements of the nuclear membrane is directed by several mechanisms.

During anaphase, many of the phosphorylated proteins in prophase began to dephosphorylate. This allows at the beginning of telophase, the nuclear vesicles begin to reassemble, associating with the surface of the chromosomes.

On the other hand the nuclear pore is reassembled allowing the pumping of nuclear proteins. The proteins of the nuclear lamina are dephosphorylated, allowing them to associate again, to complete the formation of said nuclear lamina.

Finally, after the chromosomes are completely decondensed, RNA synthesis is restarted, forming the nucleoli again and thus completing the formation of the new interphase nuclei of the daughter cells.

Cytokinesis

Cytokinesis is taken as a separate event from nuclear division, and commonly in typical cells, the cytoplasmic division process accompanies each mitosis, beginning at anaphase. Several studies have shown that in some embryos, multiple nuclear divisions occur prior to cytoplasmic division.

The process begins with the appearance of a groove or cleft that is marked in the plane of the metaphase plate, ensuring that the division occurs between groups of chromosomes. The site of the cleft is indicated by the mitotic spindle specifically, the microtubules of the asters.

In the marked cleft a series of microfilaments are found forming a ring directed towards the cytoplasmic side of the cell membrane, largely composed of actin and myosin. These proteins interact with each other allowing the ring to contract around the groove.

This contraction is generated by the sliding of the filaments of these proteins, when interacting with each other, in the same way as they do, for example, in muscle tissues.

The contraction of the annulus deepens, exerting a "clamping" effect that finally divides the progenitor cell, allowing the separation of the daughter cells, with their developing cytoplasmic contents.

Cytokinesis in plant cells

Plant cells have a cell wall, so their cytoplasmic division process is different from that previously described and begins in telophase.

The formation of a new cell wall begins when the microtubules of the residual spindle are assembled, constituting the fragmoplast. This cylindrical structure is made up of two sets of microtubules that are connected at their ends, and whose positive poles are embedded in an electronic plate in the equatorial plane.

Small vesicles from the Golgi apparatus, packed with cell wall precursors, travel through the microtubules of the fragmoplast to the equatorial region, combining to form a cell plate. The contents of the vesicles are secreted into this plate as it grows.

This plaque grows, merging with the plasma membrane along the cell perimeter. This occurs due to the constant rearrangement of the microtubules of the fragmoplast at the periphery of the plate, allowing more vesicles to move towards this plane and empty their contents.

In this way, the cytoplasmic separation of the daughter cells occurs. Finally, the content of the cell plate, together with the cellulose microfibers within it, allows the formation of the new cell wall to be completed.

Features

Mitosis is a mechanism of division in cells, and is part of one of the phases of the cell cycle in eukaryotes. In a simple way, we can say that the main function of this process is the reproduction of a cell in two daughter cells.

For unicellular organisms, cell division means the generation of new individuals, while for multicellular organisms this process is part of the growth and correct functioning of the entire organism (cell division generates the development of tissues and maintenance of structures).

The mitosis process is activated according to the requirements of the body. In mammals, for example, red blood cells (erythrocytes) begin to divide, forming more cells, when the body needs better oxygen uptake. Similarly, white blood cells (leukocytes) reproduce when it is necessary to fight an infection.

In contrast, some specialized animal cells practically lack the process of mitosis or it is very slow. Examples of this are nerve cells and muscle cells).

In general, they are cells that are part of the connective and structural tissue of the body and whose reproduction is necessary only when a cell has a defect or deterioration and needs to be replaced.

Regulation of cell growth and division.

The cell division and growth control system is much more complex in multicellular organisms than in unicellular ones. In the latter, reproduction is basically limited by the availability of resources.

In animal cells, division is arrested until there is a positive signal to activate this process. This activation comes in the form of chemical signals from neighboring cells. This allows to prevent the unlimited growth of tissues, and the reproduction of defective cells, which can seriously harm the life of the organism.

One of the mechanisms that control cell multiplication is apoptosis, where a cell dies (due to the production of certain proteins that activate self-destruction) if it presents considerable damage or is infected by a virus.

There is also the regulation of cell development through the inhibition of growth factors (such as proteins). Thus the cells remain at the interface, without proceeding to the M phase of the cell cycle.

Organisms that carry it out

The process of mitosis takes place in the vast majority of eukaryotic cells, from unicellular organisms such as yeast, which use it as an asexual reproduction process, to complex multicellular organisms such as plants and animals.

Although in general, the cell cycle is the same for all eukaryotic cells, there are notable differences between unicellular and multicellular organisms. In the former, the growth and division of cells is favored by natural selection. In multicellular organisms, proliferation is limited by strict control mechanisms.

In unicellular organisms, reproduction occurs in an accelerated way, since the cell cycle operates constantly and the daughter cells quickly embark on mitosis to continue with this cycle. Whereas the cells of multicellular organisms take considerably longer to grow and divide.

There are also some differences between the mitotic processes of plant and animal cells, as in some of the phases of this process, however, in principle, the mechanism operates in a similar way in these organisms.

Cell division in prokaryotic cells

Prokaryotic cell

Prokaryotic cells generally grow and divide at a faster rate than eukaryotic cells.

Organisms with prokaryotic cells (generally unicellular or in some cases multicellular) lack a nuclear membrane that isolates the genetic material within a nucleus, so it is dispersed in the cell, in an area called the nucleoid. These cells have a circular main chromosome.

Cell division in these organisms is therefore much more direct than in eukaryotic cells, lacking the mechanism described (mitosis). In them, reproduction is carried out by a process called binary fission, where DNA replication begins at a specific site on the circular chromosome (origin of replication or OriC).

Two origins are then formed that migrate to opposite sides of the cell as replication occurs, and the cell is stretched to twice its size. At the end of replication, the cell membrane grows into the cytoplasm, dividing the progenitor cell into two daughters with the same genetic material.

Evolution of mitosis

The evolution of eukaryotic cells brought with it an increase in complexity in the genome. This involved the development of more elaborate division mechanisms.

What preceded mitosis?

There are hypotheses that propose that bacterial division is the predecessor mechanism of mitosis. A certain relationship has been found between proteins associated with binary fission (which may be those that anchor chromosomes to specific sites on the plasma membrane of daughters) with tubulin and actin in eukaryotic cells.

Some studies point to certain peculiarities in the division of modern unicellular protists. In them the nuclear membrane remains intact during mitosis. The replicated chromosomes remain anchored to certain sites on this membrane, separating when the nucleus begins to stretch during cell division.

This shows some coincidence with the process of binary fission, where the replicated chromosomes attach to certain places on the cell membrane. The hypothesis then suggests that the protists that present this quality during their cell division could have maintained this characteristic of an ancestral prokaryotic cell.

At present, explanations have not yet been developed as to why in eukaryotic cells of multicellular organisms it is necessary for the nuclear membrane to disintegrate during the process of cell division.

References

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