CELL DIVISION: TYPES, PROCESSES AND IMPORTANCE - BIOLOGY - 2025

The cell division is the process that allows all living organisms grow and reproduce. In prokaryotes and eukaryotes, the result of cell division is daughter cells that possess the same genetic information as the original cell. This happens because, prior to division, the information contained in DNA is duplicated.

In prokaryotes, division occurs by binary fission. The genome of most prokaryotes is a circular DNA molecule. Although these organisms do not have a nucleus, DNA is in a compact form called a nucleoid, which differs from the cytoplasm that surrounds it.

Source: Retama

In eukaryotes, division occurs through mitosis and meiosis. The eukaryotic genome consists of large amounts of DNA organized within the nucleus. This organization is based on the packaging of DNA with proteins, forming chromosomes, which contain hundreds or thousands of genes.

The very diverse eukaryotes, both unicellular and metazoan, have life cycles that alternate between mitosis and meiosis. These cycles are those with: a) gametic meiosis (animals, some fungi and algae), b) zygotic meiosis (some fungi and protozoa); and c) alternation between gametic and zygotic meiosis (plants).

Types

Cell division can be by binary fission, mitosis, or meiosis. Each of the processes involved in these types of cell division is described below.

Binary fission

Prokaryotic fission, binary fission, is a form of asexual reproduction.

Binary fission consists of the division of the cell that gives rise to two daughter cells, each with an identical copy of the DNA of the original cell.

Before the division of the prokaryotic cell, DNA replication takes place, which begins at a specific site on the double-stranded DNA, called the origin of replication. Replication enzymes move in both directions from the origin, producing one copy of each of the strands of double-stranded DNA.

After DNA replication, the cell elongates and the DNA is separated within the cell. Immediately, a new plasma membrane begins to grow in the middle of the cell, forming a septum.

This process is facilitated by the FtsZ protein, which is evolutionarily highly conserved in prokaryotes, including Archaea. Finally, the cell divides.

The cell cycle and mitosis

The stages a eukaryotic cell goes through from between two successive cell divisions is known as the cell cycle. The duration of the cell cycle varies from a few minutes to months, depending on the type of cell.

The cell cycle is divided into two stages, namely the M phase and the interface. Two processes occur in the M phase, called mitosis and cytokinesis. Mitosis consists of nuclear division. The same number and types of chromosomes present in the original nucleus are found in the daughter nuclei. Somatic cells of multicellular organisms divide by mitosis.

Cytokinesis consists of the division of the cytoplasm to form daughter cells.

The interface has three phases: 1) G1, cells grow and spend most of their time in this phase; 2) S, genome duplication; and 3) G2, replication of mitochondria and other organelles, condensation of chromosomes, and assembly of microtubules, among other events.

Stages of mitosis

Mitosis begins with the end of the G2 phase, and is divided into five phases: prophase, prometaphase, metaphase, anaphase, and telophase. They all happen continuously.

Prophase

Prophase. Leomonaci98, from Wikimedia Commons

At this stage the assembly of the mitotic spindle, or mitotic apparatus, is the main event. The prophase begins with the compaction of the chromatin, forming the chromosomes.

Each chromosome has a sister chromatid pair, with identical DNA, that are tightly bound in the vicinity of their centromeres. Protein complexes called cohesins participate in this union.

Each centromere is attached to a kinetochore, which is a complex of proteins that binds to microtubules. These microtubules allow each copy of the chromosomes to be transferred to the daughter cells. Microtubules radiate from each end of the cell and form the mitotic apparatus.

In animal cells, prior to prophase, the centrosome duplication occurs, which is the main organizing center for microtubules and the place where the parent and child centrioles meet. Each centrosome reaches the opposite pole of the cell, establishing a bridge of microtubules between them called the mitotic apparatus.

In more recently evolved plants, unlike animal cells, there are no centrosomes and the origin of the microtubules is unclear. In photosynthetic cells of older evolutionary origin, such as green algae, there are centrosomes.

Prometaphase

Leomonaci98

Mitosis must ensure the segregation of the chromosomes and the distribution of the nuclear envelope of the nuclear pore complex and nucleoli. Depending on whether the nuclear envelope (EN) disappears or not, and the degree of densintegration of the EN, mitosis ranges from closed to completely open.

For example, in S. cerevisae the mitosis is closed, in A. nidulans it is semi-open, and in humans it is open.

In closed mitosis, the polar bodies of the spindle are found within the nuclear envelope, constituting the nucleation points of nuclear and cytoplasmic microtubules. The cytoplasmic microtubules interact with the cell cortex, and with the kinetochores of the chromosomes.

In semi-open mitosis, because the EN is partially disassembled, the nuclear space is invaded by nucleated microtubules from the centrosomes and through two openings in the EN, forming bundles surrounded by the EN.

In open mitosis, complete disassembly of the EN occurs, the mitotic apparatus is completed, and the chromosomes begin to be displaced toward the middle of the cell.

Metaphase

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

In metaphase, the chromosomes line up at the equator of the cell. The imaginary plane perpendicular to the axis of the spindle, passing through the inner circumference of the cell, is called the metaphase plate.

In mammalian cells, the mitotic apparatus is organized into a central mitotic spindle and a pair of asters. The mitotic spindle consists of a bilateral symmetrical bundle of microtubules that is divided at the equator of the cell, forming two opposite halves. Asters are made up of a group of microtubules at each pole of the spindle.

In the mitotic apparatus there are three groups of microtubules: 1) astral, which form the aster, start from the centrosome and radiate towards the cell cortex; 2) of the kinetochore, which are attached to chromosomes through the kinetochore; and 3) polar, which interdigitate with microtubules from the opposite pole.

In all of the microtubules described above, the (-) ends face the centrosome.

In plant cells, if there is no centrosome, the spindle is similar to that of animal cells. The spindle consists of two halves with opposite polarity. The ends (+) are on the equatorial plate.

Anaphase

Source: Leomonaci98, from Wikimedia Commons

Anaphase is divided into early and late. In early anaphase, separation of sister chromatids occurs.

This separation occurs because the proteins that maintain the union are cleaved and because there is a shortening of the microtubules of the kinetochore. When the pair of sister chromatids separate, they are called chromosomes.

During the poleward shift of chromosomes, the kinetochore moves along the microtubule of the same kinetochore as its (+) end dissociates. Because of this, the movement of chromosomes during mitosis is a passive process that does not require motor proteins.

In late anaphase, a greater separation of the poles occurs. A KRP protein, attached to the (+) end of polar microtubules, in the region of overlap of the same, travels towards the (+) end of an adjacent antiparallel polar microtubule. Thus, KRP pushes the adjacent polar microtubule toward the (-) end.

In plant cells, after the separation of chromosomes, a space with interdigitated, or superimposed, microtubules remains in the middle of the spindle. This structure allows the start of the cytokinetic apparatus, called the fragmoplast.

Telophase

Telophase. Leomonaci98

In telophase, various events happen. Chromosomes reach the poles. The kinetochore disappears. The polar microtubules continue to elongate, preparing the cell for cytokinesis. The nuclear envelope is re-formed from fragments of the mother envelope. The nucleolus reappears. The chromosomes are decondensed.

Cytokinesis

Cytokinesis is the phase of the cell cycle during which the cell divides. In animal cells, cytosinesis occurs by means of a constriction belt of actin filaments. These filaments slide past each other, the diameter of the strap decreases, and a cleavage groove forms around the circumference of the cell.

As the constriction continues, the sulcus deepens and an intercellular bridge is formed, which contains the midbody. In the central region of the intercellular bridge are the bundles of microtubules, which are covered by an electrodense matrix.

The breakdown of the intercellular bridge between post-mitotic sister cells occurs through abscission. There are three types of abscission: 1) mechanical breakdown mechanism; 2) mechanism of filling by internal vesicles; 3) constriction of the plasma membrane for fission.

In plant cells, membrane components assemble within them and the cell plate is formed. This plaque grows until it reaches the surface of the plasma membrane, fusing with it and dividing the cell in two. Then the cellulose is deposited on the new plasma membrane and forms the new cell wall.

Meiosis

Meiosis is a type of cell division that reduces the number of chromosomes in half. Thus, a diploid cell divides into four haploid daughter cells. Meiosis occurs in germ cells and gives rise to gametes.

The stages of meiosis consist of two divisions of the nucleus and cytoplasm, namely meiosis I and meiosis II. During meiosis I, the members of each pair of homologous chromosomes separate. During meiosis II, the sister chromatids separate and four haploid cells are produced.

Each stage of mitosis is divided into prophase, prometaphase, metaphase, anaphase, and telophase.

Meiosis I

- Prophase I. Chromosomes condense and the spindle begins to form. DNA has doubled. Each chromosome is made up of sister chromatids, attached to the centromere. Homologous chromosomes pair during synapse, allowing crossing over, which is key to producing different gametes.

- Metaphase I. The pair of homologous chromosomes line up along the metaphase plate. The chiasm helps hold the pair together. Microtubules of the kinetochore at each pole bind to a centromere of a homologous chromosome.

- Anaphase I. The microtubules of the kinetochore are shortened and the homologous pairs are separated. One duplicate homolog goes to one pole of the cell, while the other duplicate homolog goes to the other side of the pole.

- Telophase I. Separate homologues form a group at each pole of the cell. The nuclear envelope re-forms. Cytokinesis happens. The resulting cells have half the number of chromosomes of the original cell.

Meiosis II

- Prophase II. A new spindle forms in each cell and the cell membrane disappears.

- Metaphase II. Spindle formation is completed. Chromosomes have sister chromatids, joined at the centromere, aligned along the metaphase plate. The microtubules of the kinetochore that start from opposite poles bind to the centromeres.

- Anaphase II. Microtubules shorten, centromeres divide, sister chromatids separate and move toward opposite poles.

- Telophase II. The nuclear envelope is formed around four groups of chromosomes: four haploid cells are formed.

Importance

Some examples illustrate the importance of different types of cell division.

- Mitosis. The cell cycle has irreversible points (DNA replication, separation of sister chromatids) and checkpoints (G1 / S). The p53 protein is key to the G1 checkpoint. This protein detects DNA damage, stops cell division, and stimulates the activity of enzymes that repair damage.

In more than 50% of human cancers, the p53 protein has mutations that nullify its ability to bind specific DNA sequences. Mutations in p53 can be caused by carcinogens, such as benzopyrene in cigarette smoke.

- Meiosis. It is associated with sexual reproduction. From an evolutionary point of view, it is believed that sexual reproduction arose as a process to repair DNA. Thus, damage to a chromosome can be repaired based on information from the homologous chromosome.

The diploid state is believed to have been transient in ancient organisms, but became more relevant as the genome got larger. In these organisms, sexual reproduction has the function of complementation, DNA repair and genetic variation.

References

1. Alberts, B., Johnson, A., Lewis, J., et al. 2007. Molecular Biology of the Cell. Garland Science, New York.
2. Bernstein, H., Byers, GS, Michod, RE 1981. Evolution of sexual reproduction: the importance of DNA repair, complementation, and variation. American Naturalist, 117, 537-549.
3. Lodish, H., Berk, A., Zipurski, SL, Matsudaria, P., Baltimore, D., Darnell, J. 2003. Cellular and molecular biology. Editorial Medica Panamericana, Buenos Aires.
4. Raven, PH, Johnson, GB, Losos, JB, Singer, SR 2005 Biology. Higher Education, Boston.
5. Solomon, BM, Berg, LR, Martin, DW 2008. Biology. Thomson, USA.