14 julio, 2024

Mitosis: what it is, phases, characteristics, functions, organisms

What is mitosis?

The mitosis It is a process of cell division, where a cell produces genetically identical daughter cells. For each cell, two “daughters” are generated with the same chromosome load. This division takes place in the somatic cells of eukaryotic organisms.

It is one of the stages of the cell cycle of eukaryotic organisms, 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 to be an interface. Nuclear and cytoplasmic division (mitosis and cytokinesis) make up the last stage of the cell cycle.

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 cell 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 parent cell and its daughters have exactly the same genetic information. These cells are known as diploid, because they carry the full chromosome load (2n).

Meiosis, on the other hand, is the process of cell division that gives rise 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 chromosome load, which are called haploid (n).

Mitosis Overview

Mitosis in unicellular organisms generally produces daughter cells that closely resemble their parents. 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 cellular 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, which 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 organism.

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 makes it possible to ensure the continuity of their genetic information.

The understanding of the processes of mitosis and meiosis has played a fundamental role in understanding the curious 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 kind of cut or injury to our skin, we see how the damaged skin recovers in a matter of days. 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 from other cells, and the division of the cytoplasm.

In some cases, nuclear division and cytoplasmic division are considered to be separate phases (mitosis and cytokinesis, respectively).

For a better understanding of the process, 6 phases were designated, called: prophase, prometaphase, metaphase, anaphase and telophase, considering cytokinesis as a sixth phase, which begins to develop during anaphase.

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

Prophase

Prophase is the first visible manifestation of cell division. In it you can see the appearance of chromosomes as distinguishable shapes, due to the progressive compaction of chromatin. This chromosome condensation begins with the phosphorylation of histone H1 molecules by MPF kinase.

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

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

Formation of the mitotic spindle

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

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

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

The duplicated centrosomes begin to separate from one another as the spindle microtubules in each assemble, beginning to migrate to 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

At this stage, the nuclear envelope has been completely fragmented, so the spindle microtubules 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 toward the center of the cell), the centrosomes, and a pair of asters (radially distributed structures of short microtubules that fan out from each centrosome).

The chromatids each developed a specialized protein structure, called a kinetochore, located at 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 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

In metaphase, the centrosomes are located at opposite ends of the 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 kinetochores of the chromatids remain attached to the kinetochore microtubules. Microtubules that do not attach 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 «birdcage» appearance.

Morphologically, this phase is the one that appears to have the least changes, which is why it came to be considered a resting phase. However, although they are not easily appreciable, many important processes occur in it, in addition to being the longest stage of mitosis.

Anaphase

During anaphase, each pair of chromatids begins to separate (by 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 kinetochore microtubules, generating a “pull” effect that causes each chromosome to move from its centromere. Depending on the location of the centromere on the chromosome, it can take a particular shape, such as V or J, during its displacement.

Microtubules not attached to the kinetochore grow and elongate by tubulin (protein) adhesion 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 spindle poles move away from each other, elongating the cell.

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

telophase

Telophase is the last phase of nuclear division. The kinetochore microtubules disintegrate while the polar microtubules elongate further.

The nuclear membrane begins to form around each set of chromosomes, using the nuclear envelopes of the parent cell, present as vesicles in the cytoplasm.

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

During anaphase, many of the proteins phosphorylated in prophase began to be dephosphorylated. This allows that at the beginning of telophase, the nuclear vesicles begin to re-assemble, associating with the surface of the chromosomes.

On the other hand, the nuclear pore reassembles allowing the pumping of nuclear proteins. Nuclear lamina proteins are dephosphorylated, allowing them to reassociate, to complete nuclear lamina formation.

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

cytokinesis

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

The process begins with the appearance of a groove or groove that is marked in the plane of the metaphase plate,…

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