BIPARTITION OR BINARY FISSION: PROCESS, TYPES AND EXAMPLES - BIOLOGY - 2025

The bipartition or binary fission is a type of asexual reproduction by which a parent, usually unicellular, divides to form two daughter cells identical size smaller from the genetic point of view (clones).

This type of reproduction is typical of bacterial species and differs from other types of asexual reproduction such as budding, for example, in the fact that the parental cell disappears, giving rise to two smaller daughter cells that have part of the cell envelope. from the first. It also exists in eukaryotic unicellular and colonial organisms such as protozoa and some unicellular algae.

Binary fission process. 1. Bacteria before binary fission with packaged DNA. 2. DNA replicates. 3. The DNA goes to the poles of the bacterium as it increases in size in preparation for division. 4. A new cell wall begins to grow. 5. The new cell wall has developed. 6. New bacterial cells have packed DNA, ribosomes, and plasmids.

Asexual reproduction differs from sexual reproduction in that it does not involve the fusion of two gametic cells with half the chromosomal load, nor does it involve the formation of new genetically different individuals.

Furthermore, asexual reproduction usually occurs in single-celled organisms, while sexual reproduction is typical of multicellular organisms.

This type of asexual reproduction ensures the formation of clones, as well as a rapid increase in the number of individuals in a given population.

Binary fission process

Binary fission is the main reproductive mechanism of prokaryotic organisms. In bacteria it begins with the duplication of the single bacterial circular chromosome and with a considerable increase in cell size.

The two copies thus produced must migrate or segregate towards the two poles of the cells, after which the proteins necessary to form the division machinery are assembled at the site where the separation of the daughter cells will occur (usually in the form of a ring).

Binary fission process in bacteria (Source: Mcstrother via Wikimedia Commons) After this complex and regulated process, a kind of transverse “wall” is formed that separates the two resulting chromosomes, said wall is formed by the internal growth of the plasma membrane and the cell wall.

When the wall is fully formed, it results in the separation of the two daughter cells.

This form of reproduction is remarkably fast, taking place under ideal conditions at intervals of around 20 minutes. This allows bacterial species to increase the number of individuals in a colony at a considerably high rate. In the following video you can see this process in protozoa:

Types of binary fission

Binary fission in prokaryotes has been classified into transverse binary fission and longitudinal binary fission, according to the axis along which the separation of daughter cells occurs.

Transverse binary fission

Transverse binary fission, as its name implies, occurs through the transverse axis of the dividing cell, in other words, the "wall" that will separate the two daughter cells is formed on the axis that is perpendicular to the axis that demarcates the greatest length of the cell.

This type of division occurs in some flatworms and polyps, where it is known as strobilation. Some texts suggest that transverse binary fission results in the formation of a kind of "strings" or chains with the fission products.

Longitudinal binary fission

Longitudinal binary fission, on the other hand, occurs when the dividing cell does so through the longitudinal axis, which is usually the one that demarcates the greatest length of a cell.

Irregular binary fission

Irregular binary fission is one by which cytosolic division or cytokinesis occurs in the plane perpendicular to that of nuclear division in eukaryotes.

Specific examples

Many unicellular algae divide asexually by binary fission, which is also true for most species of protozoa, although species in both groups take advantage of both sexual and asexual types of reproduction.

In paramecia

Parameciums are organisms of the group of protozoa that usually live in bodies of stagnant water rich in organic matter.

They are eukaryotic microorganisms that have a shape similar to that of a sandal or the sole of a shoe and are covered by cilia that serve as locomotor “organs” or “appendages”.

Types of division in ciliates (Source: Deuterostome via Wikimedia Commons) The main reproductive mechanism of this group of invertebrates is transverse binary fission, although they can also reproduce sexually. However, the binary fission process in these organisms is somewhat different, since the replication of nuclear genetic material occurs by mitosis. In the following video you can see the bipartition in paramecia:

In amoebas

Amoebas are also invertebrate eukaryotic organisms belonging to the group of protozoa. They do not have a defined body shape and move by projecting portions of their body called pseudopods.

Like paramecia, the replication of the genome of amoebae that divide by binary fission involves a mitotic event.

In bacteria

Life cycle of the bacterium Myxococcus xanthus.. It reproduces by binary fission. Elena García Bravo Bacteria are, without a doubt, the most representative group that reproduces by bipartition or binary fission, and it is where this process has been studied in greater detail.

Among the best-known species of this group of eukaryotic organisms are Escherichia coli, a gram-negative enterobacterium naturally present in the intestinal flora of humans, and Caulobacter crescentus, another gram-negative (non-pathogenic) bacterium present in many soils and bodies of water.

The gram-positive bacterium Bacillus subtilis, which is a typical bacterium found in many soils, has also served as a study model.

Differences between binary fission and mitosis

The bipartition process is somewhat different from typical mitotic events in eukaryotic cells. First, during binary fission the mitotic spindle that separates the chromatids during mitosis is not produced. There is also no duplication of organelles, as this process is typical of bacteria, which lack internal membranous organelles.

Another important difference is that bipartition occurs in bacteria and other microorganisms with a view to increasing the number of individuals, while in multicellular organisms such as animals and plants, for example, this process serves to replace cells., growth and development.

Last but not least, it is important to establish that mitosis takes much longer than binary fission, which is because it is a slightly more complex and costly process from an energy point of view.

References

1. Angert, ER (2005). Alternatives to binary fission in bacteria. Nature Reviews Microbiology, 3 (3), 214.
2. Brusca, RC, & Brusca, GJ (2003). Invertebrates (No. QL 362. B78 2003). Basingstoke.
3. Prescott, LM, Harley, JP, & Klein, DA (1993). Microbiology, 2nd edn. Wm. C. Brown. Pub., New York, 224.
4. Simpson, JY (1902). Observations on binary fission in the life-history of Ciliata. Proceedings of the Royal Society of Edinburgh, 23, 401-421.
5. Solomon, EP, Berg, LR, & Martin, DW (2011). Biology (9th edn). Brooks / Cole, Cengage Learning: USA.