Binary fission

Binary fission definition

What is binary fission? In biology, binary fission is a type of asexual reproduction where a parent cell divides, resulting in two identical cells, each having the potential to grow to the size of the original cell. The word asexual describes a reproduction that occurs without involving sex cells (gametes). Instead, the somatic cells undergo an asexual process that will produce a clone of the parent. The offspring is a clone because its genome will be identical to that of the parent. Binary fission is common among prokaryotes, e.g. archaea, eubacteria, cyanobacteria, and certain protozoans (e.g. amoeba).

Process of binary fission

How does binary fission work? Binary fission is the way that prokaryotic cells and certain protozoans reproduce. Similar to other modes of asexual reproduction, such as budding and formation of baeocytes (e.g. in cyanobacterium Stanieria)¹, the result is an offspring that has the same genome as the parent.

Prokaryotic binary fission

The prokaryotic cell contains DNA that is tightly coiled prior to cellular splitting. The process starts by creating a replicate of the genetic material. Next, the chromosomes segregate to separate poles of the cell — a process called “karyokinesis”. The cytoplasm is subsequently cleaved into two by a new cell membrane forming (cytokinesis). A cell wall also forms if the original (parent) cell has one. The new cell wall often starts out as a “Z-ring” as formed by the cytoskeleton FtsZ. Below is a diagram depicting a bacterium undergoing cell fission. As shown, the method of bacterial replication appears to be fast and simple. Inside a bacterial cell, the stages are as follows: (1) genomic replication, (2) chromosome segregation, and (3) cytokinesis.

Binary fission in eukaryotic cells

Sexual reproduction is common among eukaryotes; however, some of them can reproduce asexually. Protozoa are examples of eukaryotes that can reproduce by binary fission. Mitochondria, one of the major organelles in eukaryotic cells, divide by binary fission. The process is similar to a prokaryotic fission. This is one of the basis of the theory called Endosymbiotic theory wherein primitive prokaryotic cells are presumed to have evolved into the mitochondria that we know today.

Types of binary fission

Binary fission has four types according to how the cell divides: (1) irregular, (2) transverse, (3) longitudinal, and (4) oblique.

Irregular

In irregular binary fission, the cell divides at any plane. However, it is mostly in a manner perpendicular to where the nuclear division (karyokinesis) occurred.

Transverse

In transverse-type, cell division occurs along the transverse axis, hence the name.

Longitudinal

In longitudinal binary fission, the cell divides longitudinally.

Oblique

In oblique binary fission, cell division occurs obliquely, which may either by left or right oblique.

Difference between binary fission and mitosis

Binary fission is similar to mitosis in the way that the process ultimately leads to the production of two identical daughter cells. However, they differ in many aspects. While binary fission is for reproductive purposes mitosis is primarily for growth in multicellular organisms. The genetic material is replicated prior to mitosis whereas this stage occurs as part of binary fission. Furthermore, spindle fibers are one of the features that define mitosis. The spindle fibers attach to the chromosomes to move and divide the chromosomes into two equal sets at opposite poles. The new cells from mitosis will not be a new individual as it is in binary fission. They will become a somatic cell that will either develop into a specialized differentiated cell or a cell that divides mitotically to give rise to another set of new cells.

Table: Binary fission vs Mitosis

Binary fission	Mitosis
Function: reproduction	Function: cell growth (in numbers) in multicellular organisms
Occurs in prokaryotes and eukaryotes	Occurs in eukaryotes
No spindle forms	Mitotic spindle forms
Stages: chromosome duplication, chromosome segregation, cytokinesis	Stages: prophase, metaphase, anaphase, telophase
Product: two cells with identical genome	Product: two cells with identical genome

Binary fission examples

Several organisms perform binary fission. Bacteria, for instance, use it as a way to reproduce. As already mentioned above, bacterial fission entails chromosomal replication, chromosomal segregation, and cell splitting. Another group of organisms that reproduce by binary fission is the protozoa. In a protozoan fission, the process is similar as it entails similar fundamental stages. However, protozoa differ from prokaryotes in having mitochondria that have to be duplicated and divided as well. They differ in how their cell splits. For instance, in amoebae, cytokinesis takes place along any plane. Thus, their binary fission exemplifies the irregular-type. As for longitudinal-type, Euglena is an example. Ceratium, in turn, is a protozoan in which cytokinesis occurs obliquely. Paramecium is an example of a protozoan whose binary fission is transverse-type.

See also

• Asexual reproduction

References

1. Binary Fission and other Forms of Reproduction in Bacteria | Department of Microbiology. (2020). Cornell.Edu. https://micro.cornell.edu/research/epulopiscium/binary-fission-and-other-forms-reproduction-bacteria/

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