What is Binary Fission in a cell? Definition, Steps, Types and Examples

What is Binary fission?

Binary fission is a kind of asexual reproduction whereby a single cell duplicates its genetic material and separates into two daughter cells that have the ability to grow into the original size of the parent cell. In binary fission, it can be a cell or organelle that splits into two and each of the new organisms or organelle formed possesses the same genetical material or DNA of the parent cell or organelle.

This fission is different from other fission in that the process involves the formation of only two cells from, a parent cell. This reproduction is asexual because it doesn’t involve the fusion of sex cells or gamete. Many prokaryotes and some eukaryotes reproduce via binary fission and some organelles in the cell such as mitochondria through the process of binary fission undergo cell division.

Typical examples of organisms that reproduce via binary fission include cyanobacteria, eubacteria, paramecium, amoeba, and archaea. Bacteria to be able to remain viable and competitive tend to divide and provide each offspring with a complete copy of their genetic material.

In binary fission, the process at which the parent cell divides into two daughter cells is known as cytokinesis. This asexual reproduction is the primary means of reproduction in prokaryotes. In this reproduction, an organism duplicates its genetic material, or deoxyribonucleic acid (DNA), and as it divides into two as each new organism receives one copy of the DNA.

Furthermore, depending on the axis of cell separation, there are about four types of binary fission. We have two types of binary fission in protists which include transverse fission and longitudinal fission. The regular transverse fission is called strobilation in some organisms like tapeworms and scyphozoan jellyfish polyps. Strobilation usually gives rise to a chain of fission products referred to as strobilus.

In tapeworms, the fission products are the proglottids while in scyphozoan jellyfish it is the ephyrae. The ephyrae and the proglottids all mature and eventually separate from the strobilus end. In certain organisms like the amoeba, fission is an irregular type because the cell separation is along any plane. Whereas, fission can be a longitudinal type in organisms like the euglena. In paramecium, fission is a transverse type and is said to be an oblique type in organisms like ceratium.

Furthermore, few metazoan species undergo body division similar to binary fission called fragmentation. Planarian fission and fragmentation generally portray direct reproduction. It involves each portion regenerating missing parts to become a completely new organism. However, strobilation products represent indirect reproduction. The proglottids, for instance, are not regenerative rather they carry and release a large number of eggs and die. Ephyrae, on the other hand, do not reproduce new polyps but mature into medusae that are sexual. It’s the larvae of the medusa that then become polyps.

Binary fission definition in biology

Binary fission can be defined in biology as an asexual reproduction that involves the separation of a parent cell in a process of cytokinesis to produce two daughter cells that receive a copy of the parent cell DNA. Many prokaryotic organisms make use of binary fission as their means of reproduction and unlike mitosis, fission takes place without the formation of any spindle apparatus in the cell.

The process of fission may be similar to mitosis but is different. In the process of fission, the DNA replicates and each copy of the DNA molecules attaches to several parts of the cell membrane. Then, the initial and replicated chromosomes pull apart once the cell begins to get drawn back.

Reproducing asexually has its disadvantage. All the cells reproduced asexually are identical genetically and are copies of each other and the parent cell. Hence, they are all susceptible to the same disease or environmental stress.  For instance, antibiotics on bacteria are illustrative of this principle. A resultant daughter cell will be vulnerable to an antibiotic that its parent cell is vulnerable to. However, if there is a mutation in their genes, then a particular strain resistant to the antibiotics can occur.

Bacteria reproduce via fission in a process that includes the utilization of FtsZ protein, chromosomal replication, segregation of the chromosome, and splitting of the cell. However, the process of cell splitting and the division of the cytoplasm may differ in protozoan organisms like paramecium, amoeba, and euglena. Generally, organisms that reproduce via binary fission include Amoeba, Bacillus subtilis, Bacillus cereus, Escherichia coli, Bascillus pumilus, Clostridium perfringens, euglena, Corynebacterium diphtheria, ceratium, paramecium, etc.

Conclusively, it is basically organisms in the domains of Archaea and Bacteria that reproduce asexually with binary fission. This type of cell division and asexual reproduction is also used by some organelles within eukaryotic organisms such as the mitochondria. Binary fission, therefore, results in the reproduction of an organelle or a living prokaryotic cell.

Binary fission steps

The prokaryotic cell has DNA that is tightly coiled before the cell splits. Here are the steps of binary fission:

1. The binary fission process begins with the replication of the genetic material.
2. Then, a process known as karyokinesis occurs whereby the chromosome of the cell segregates to separate the cell poles.
3. Cytokinesis takes place whereby as a new cell membrane forms, the cytoplasm of the cell is divided into two.
4. The cytokinesis is followed by the formation of a cell wall if the parent cell had a cell wall and a new cell wall usually begin as a Z-ring which is formed by the cytoskeleton FtsZ. In bacteria, the FtsZ is the first protein to confine to the site of future division and it assembles into a Z-ring that is anchored by FtsZ-binding proteins.
5. FtsZ is comparative to β-tubulin that is the building block of the microtubule cytoskeleton which is used in eukaryotes during mitosis. Z-ring defines the division plane between the two daughter cell.
6. Min proteins; MinC and MinD work together as division inhibitors and block the formation of the FtsZ ring whereas, MinE puts a stop to the activity of the MinCD midcell and allows FtsZ to take over for binary fission.

Steps to binary fission in bacteria

Here are more specific binary fission steps that occur in bacteria:

1. Replication of DNA: Prior to fission, the DNA of the bacterium is tightly coiled. Then, the DNA of the bacterium uncoils and duplicates, and as this chromosome replicates it doubles its content.
2. Cell growth: The DNA is pulled to opposite poles of the bacterial cell, increasing its size as it prepares for fission. This is followed by an increase in the cytoplasmic content.
3. Segregation of DNA: As the cell elongates, a new cell wall forms in the middle. The growth of this new cell wall (septum) is triggered by the FtsZ polymerization and the formation of Z-ring begins to separate the bacterium into two. It is in this phase that the two chromosomes are separated.
4. Splitting of the cell: Apparently, the new cell wall develops fully and result in the complete division of the bacterium. The new daughter cells as newly produced organisms possess tightly coiled DNA rods, plasmids, and ribosomes. Therefore they each contain a copy of the nuclear material as their necessary organelles.

However, there are bacteria known as the L-form bacteria that do not produce a cell wall. Studies have shown that FtsZ needs a cell wall to work. Hence, there is little knowledge on how these L-form bacteria divide though it is said to resemble the L-form’s budding-like division process of separation and extrusion.

Generally, binary fission is rapid even though its speed varies among species. For example, cells divide in E.coli almost every 20 minutes at 37°C. The new cells on their own will eventually undergo binary fission and the time needed for binary fission is the same time the bacterial culture needs to double up the number of cells it contains. This time period is therefore called the doubling time.

Some species other than E.coli, however, may have a doubling time that is much slower or faster. For instance, some strains of mycobacterium tuberculosis may have a doubling time that is at least 24 hours. Nevertheless, there are factors that limit bacterial growth such as the availability of space, and nutrients. Hence, once bacterial culture enters the stationary phase of growth, binary fission takes place at a much lower rate.

Binary fission diagram

Examples of Binary fission

There are several examples of organisms that carry out binary fission. A typical example is the bacteria as explained earlier that reproduce via binary fission. The protozoa are another group of organisms that reproduce by binary fission.

The process in protozoan fission is similar to bacterial fission that entails chromosomal replication, segregation of the chromosome, and splitting of the cell. However, these protozoan organisms differ from prokaryotes in that they have mitochondria that duplicate and divide.

They also differ in the way their cell splits. In amoeba, for example, cytokinesis occurs along any plane and thus exemplifies the irregular type of fission. Euglena, on the other hand, exhibits a longitudinal type of fission and in ceratium, cytokinesis takes place obliquely. An example of a protozoan that exhibits a transverse type of fission is the paramecium.

Binary fission in prokaryotes

Reproduction in prokaryotes is one of the examples of binary fission. Prokaryotes are single-celled organisms that are devoid of a well-defined nucleus e.g archaea and bacteria. Instead, their genetic material occurs in a nucleoid region that they possess in their cytoplasm. The genetic material of prokaryotic organisms occurs as a single circular molecule of DNA.

In human cells, the genetic material is present in the nucleus of the cell which is different for prokaryotes. Prokaryotes have their genetic material (chromosomes) as a nucleoid in a specialized region of the cell in the cytoplasm. The chromosomes undergo replication which starts from a point in the chromosome referred to as the origin of replication. This origin eventually divides and the replication continues.

The two origins from the division then move towards the opposite ends of the cell and pull the chromosomes along with them. Once replication is completed, different protein types that work as cell division machinery assemble at the center of the cell. The FtsZ protein happens to be one of the essential proteins in this process that forms a ring-like structure at the cell’s center. This process is crucial in order to make sure the division doesn’t damage the genetic material but splits the cytoplasm.

As the cytoplasm divides, the membrane pinches inward to form a new cell wall (septum) down the middle of the cell. Lastly, the new cell wall divides itself to form two new independent cells. However, an organism like Crenarchaeota that doesn’t have a cell wall or FstZ protein uses a primitive mechanism called the ESCRT-III system that involves different proteins.

In summary, the process of fission in prokaryotes involves a single DNA molecule of prokaryotic organisms that replicate first and attaches each copy to different parts of the cell membrane. Then, the replicated chromosomes separate as the cell begins to pull apart. The consequence of fission in prokaryotes is that all the cells are genetically identical. Hence, they all have the same genetic material.

Fission takes place without the formation of spindle apparatus on the cell compared to the process of mitosis and meiosis that eukaryotic cells use. For instance, the rental identity gets lost in mitosis which is not the case with binary fission.

Binary fission in archaea

Crenarchaeota doesn’t have a cell wall or the FtsZ mechanism. Hence, they make use of a primitive mechanism called the ESCRT-III system (or Cdv) that involves different proteins. This system manipulates the membrane of the cell into separating by coming in between the two impending daughter cells. Euryarchaeota, on the other hand, like bacteria, uses FtsZ.

Binary fission in bacteria

Generally, binary fission is rapid in bacteria even though its speed varies among species. The new cells on their own will eventually undergo binary fission and as a result, the time needed for fission is the same time the bacterial culture needs to double up the number of cells it contains. This time period is the doubling time.

Nevertheless, there are factors that are specific to certain species for bacterial growth such as pH levels, oxygen, moisture, osmotic pressure, and light. For example, mesophiles flourish very well at moderate temperatures that range between 20°C-45°C. Hence, many disease-causing bacteria in humans are mesophiles because the ambient temperature of the human body is 37°C. On the other hand, are the extremophiles that can survive conditions that are extremely harsh. These bacteria flourish well in conditions with high salinity, high temperature, high acidic environments, and other harsh conditions.

The rate of fission varies among bacteria species. For instance, Mycobacterium tuberculosis that causes tuberculosis in humans can divide every 15-20 hours. This is relatively slow compared to E.coli which divides every 20 minutes at 37 °C. Also, take the extremophilic Deinococcus radiodurans for instance, which can survive a thousand times more radiation than a human can. This bacterium can divide every 48 hours under normal circumstances. However, harsh conditions like drought can slow down the growth rate of the Deinococcus radiodurans until there is a change to a more favorable condition.

There is also a case of multiple fission in bacteria even though most bacteria species undergo binary reproduction. Some species of bacteria undergo multiple fission that begins or end with spores production. For example, the species Metabacterium polyspora, has been observed in each division producing multiple endospores. Also, some species of cyanobacteria have been observed to reproduce through multiple fission.

Binary fission in Eukaryotes

The majority of eukaryotes reproduce sexually. However, some of them like the amoeba reproduce asexually using binary fission. Also, some organelles like the mitochondria and chloroplasts in eukaryotic cells divide via binary fission. Eukaryotes are those unicellular or multicellular organisms that possess cells with a nucleus that is distinct and membrane-bound.

The process of binary fission in eukaryotic organisms involves the FtsZ which is the same process as that of the prokaryotic organisms. Organelles like chloroplasts and mitochondria in the cell divide via binary fission. This gives the basis for the endosymbiotic theory that explains that prokaryotic organisms have evolved into today mitochondria.

Fission in Amoeba

Amoeba as bacteria reproduces through fission. The genetic material of the bacteria replicates through mitosis. Then, after replication, the bacterial cell divides into two daughter cells that are of equal size.

Through fission, two similar individual amoebae are produced from a single-parent amoeboid cell. The amoeba grows larger when it’s about to undergo division and the nucleus of the amoeba then extends and divides into two. After the division of the nucleus, the cytoplasm divides. Therefore, two amoebae are produced from the parent amoeboid cell and the identity of the parent is technically lost.

Fission of Organelles

Some organelles such as chloroplasts and mitochondria in eukaryotic cells divide via binary fission. The fission of Mitochondria usually occurs within the cell. It takes place even when the cell is not actively undergoing mitosis, and in regard to the regulation of the cell’s metabolism, it is very necessary.

Some mitochondria that are not in animals and all chloroplasts derived from endosymbiosis of bacteria also utilize FtsZ as the bacteria does. According to scientists, the chloroplast found in the plant cell comes through endosymbiosis that involved a eukaryotic mitochondrial cell.

Binary fission in protists

In many protists such as algae and sporozoans, multiple fission occurs at the cellular level. Through a process of amitosis, several nuclei are produced as the nucleus of the parent cell divides several times. Then, multiple daughter cells are formed as the cytoplasm separates.

In order for a single parent cell of some parasitic single-celled organisms to produce numerous daughter cells, they have to undergo multiple fission-like processes. For example, the cells of the Trypanosoma borreli, that parasitize fish have been observed to partake in both multiple and binary fission. Also, the human parasite, Blastocystis hominis were observed to partake in multiple fission within 4-6 days.

Fission in apicomplexans

Multiple fission or schizogony is exhibited in the apicomplexans which is a phylum of parasitic protists. Multiple fission is exhibited either as sporogony which results in sporozoites, or gametogony that results in microgametes, or merogony that results in merozoites. Merozites are the multiple daughter cells that arise within the same membrane of the cell.

Fission in green algae

Binary fission in green algae involves the division of the algal cell into more than two daughter cells. The number of daughter cells produced is based on the algae species which is an effect of light and temperature.

Types of binary fission

• Irregular binary fission
• Transverse binary fission
• Longitudinal binary fission
• Oblique binary fission

Binary fission is grouped into four types based on the plane at which the cytoplasm is divided:

Irregular type

This type of fission occurs in organisms like the amoeba. The division of the cytoplasm (cytokinesis) in irregular binary fission takes place in any plane although it usually occurs perpendicular to the plane of karyokinesis (a division of the chromosomes).

Transverse type

This binary fission type is exhibited by protozoans like paramecium. In transverse binary fission, the division of the cytoplasm takes place along the transverse axis of the cell.

Longitudinal type

This type of binary fission takes place in flagellates such as euglena. In this type of fission, the cytoplasm divides along the longitudinal axis of the cell.

Oblique type

This type of fission takes place in dinoflagellates like ceratium where the cytoplasm divides and occurs obliquely in the left and right oblique.

Binary fission vs Mitosis

Mitosis and binary fission may be similar but at the same time differ. They are similar in that the process of fission gives rise to two identical daughter cells like in mitosis. Mitosis, however, is mainly for the purpose of growth in multicellular organisms whereas, binary fission is mainly for reproductive purposes.

Furthermore, the spindle fibers involved in mitosis are one of the defining features in contrast with fission. The spindle fibers attach to the chromosomes to move and divide them into two parts of equal size at opposite poles. However, this spindle fiber is not involved in binary fission as in mitosis.

The new cells produced from binary fission are new individuals whereas, in mitosis, they become somatic cells. This somatic cell in mitosis develops either into a specialized differentiated cell or a cell that divides through mitosis to produce another set of new cells.