All living organisms are capable of reproducing offspring to secure the survival of their species. Less complex species use methods of asexual reproduction as a means of passing along its genes. One such method is called binary fission.
Binary fission is a common asexual reproduction process that occurs in single-celled organisms such as bacteria. The process involves DNA replication and cytoplasmic division via cytokinesis to result in multiple genetic copies of the original cell.
In this article, a general overview of binary fission will be presented as well as an in-depth look into some of the genetic machinery required for binary fission to occur. It will also discuss the evolution of binary fission as it relates to alternate cell division systems within various organisms.
Fission In Biology
Whenever a single-celled or unicellular organism divides into two or more parts, they are capable of regenerating into independent organisms that replicate the original organism. It is known as fission. The original “parent” cell will copy its DNA and then form a wall-like structure to split into “daughter” cells. These genetic reproductions of the original organism go on to survive and reproduce independently.
While there are a few types of fission, the main two are binary fission and multiple fission.
Binary Fission vs. Multiple Fission
Both binary and multiple fissions are seen in bacteria. Both are considered methods in asexual reproduction, and each process has a single parent cell that replicates into daughter cells, which can then grow into independent organisms.
The main difference between binary fission and multiple fission is in the number of daughter cells that are reproduced in the process. When the organism divides into more than two parts that resemble the original, it is multiple fission.
However, when division results in only two copies that are identical, it is binary fission. Besides, multiple fission works to help grow or repair an organism, whereas binary fission is primarily a method of reproduction to expand the species. This table gives you a quick comparison of these two common types of fission.
| Binary Fission | Multiple Fission |
|---|---|
| Parent cell divides only once | Parent nucleus divides into many parts as part of mitosis |
| Parent cell produces two daughter cells | Parent cell produces multiple daughters |
| Occurs is favorable conditions | Can occur in adverse conditions |
| Protective cyst is not formed around the organism | Protective cyst formation is common |
| Organism achieves bio-indefinite mortality during the process | Immortality is absent |
| No residue is left behind | Leaves behind a residue |
| Commonly seen occurring in bacteria and Archaea | Commonly seen occurring in protista (protozoans and algae) |
Advanced Bio Extra Credit
There are three other types of fission we see in organisms, which are plasmotomy, clonal fragmentation, and population fission.
- Plasmotomy involves a multinucleate parent cell that produces multinucleate daughter cells.
- Clonal fragmentation is when multicellular organisms split into fragments that each develop into independent and functioning clones of the original.
- Population fission refers to a single population spitting off to a different location. An example of such fission could be migration. It is believed this kind of fission is the underlying reason for speciation.
Why Is Binary Fission So Significant?
Binary fission is an asexual process through which unicellular organisms can quickly reproduce and carry on their genes. It is similar to mitosis; however, the process is much simpler and faster. It is the primary method seen in prokaryotes to reproduce, but it has also been observed in the duplication of organelles in eukaryotes.
Prokaryotes and Eukaryotes: A Quick Review
There are two main classifications for all cells on earth: prokaryotes and eukaryotes. Most notably, prokaryotes have a sort of free-floating genetic material, while eukaryotes contain theirs within a distinct nucleus.
Prokaryotes
Prokaryotes are simple, unicellular organisms. They are the most primitive forms of early life that exist on earth and can be found almost everywhere. Most prokaryotes are considered to be extremophiles, which means they can survive and flourish in various environmental extremities. This includes hot springs, swamps, and even inside the guts of mammals.
In fact, prokaryotic bacteria live on the skin and in the body as part of the human microbiota.
They are lacking in a noticeable nucleus and organelles, which is what distinguishes them from eukaryotes. The membranes of prokaryotic cells are composed of phospholipids that line up to form a semi-permeable barrier around the cytoplasm that holds the DNA.
Usually, this is just one chromosome that is circular in shape, although we have found a few prokaryotes containing two chromosomes, and recent studies have even found some containing as many as four.
The cytoplasm generally contains some ribosomes, and there may be an additional DNA molecule present called a plasmid. These provide further cell functions, such as protein-encoding.
The most commonly known prokaryotes are bacteria. The three-domain system further identifies archaea as a branch of prokaryotic cells as well. There is still much to be known about archaea, but we have seemingly identified them as a middle ground between bacteria and eukaryotes, suggesting it is a step in the evolution of prokaryotic organisms into eukaryotic organisms.
In general, prokaryotes will reproduce via binary fission. Even exceptional bacteria like cyanobacteria, which is capable of performing other methods of asexual reproduction such as multiple fission or fragmentation, will commonly replicate with binary fission. It is the most efficient way for these cells to reproduce since they are complete organisms by themselves.
Eukaryotes
Eukaryotes are complex organisms. Most life forms large enough to see–plants, fungi, animals–are multicellular eukaryotes. However, most eukaryotes are microscopic unicellular organisms called protists–a classification covering anything we don’t generally recognize as a plant or animal.
Eukaryotic organisms actually represent a small portion of the population as far as living organisms go, but since they are so much larger in size, their collective biomass worldwide is said to equal that of all prokaryotes.
As mentioned previously, the distinction between prokaryotes and eukaryotes is that eukaryotes have a distinct nucleus, organelles, and many linear chromosomes that are bound by a plasma membrane. Another distinguishing feature eukaryotes have from prokaryotes is that they can reproduce sexually.
This occurrence is known as meiosis. However, asexual reproduction is also observed as a means of cell division, though it is through mitosis rather than binary fission.
Organelles
Organelles are often found inside most eukaryotes. These special structures each have their own purpose and function to carry out in the cell to support the complex system. Organelles function similarly for the cell the way organs do for the body, hence their name. A well-known example is mitochondria.
While they are not independent organisms, organelles also reproduce via binary fission. We’ll discuss this further a little later. Endosymbiotic theory tells us that organelles such as mitochondria and chloroplasts have evolved from once independent organisms to live within other cells, and it is for this reason that they still replicate through binary fission. We see this suggested in the three-domain system. Later we will take a closer look into this evolution.
An Overview Of Binary Fission
Generally, the result of binary fission produces cells that have the same genetic identity because they contain the same genetic material (provided there were no random mutations that occurred).
But this process can happen quickly, and therefore is efficient. Also, binary fission leaves the identity of the parent cell preserved, which can be helpful in rapid population growth; however, it leaves very little room for genetic modification or evolution.
The processes of mitosis and meiosis, on the other hand, require a formation on the eukaryotic cell of a spindle apparatus, which adds to the amount of time these processes need to complete but ensures a higher success rate in the accuracy of DNA replication.
Overall, there are four ways in which binary fission may occur:
- Longitudinal – along the longitudinal axis.
- Transverse – along the transverse axis.
- Irregular – on a plane perpendicular to the plane of the division in the nucleus.
- Oblique – a slightly varied version of the common process.
The Process Of Binary Fission
This process starts at the nucleoid, which is a special region inside the bacteria containing its single, circular chromosome of genetic material. The DNA starts copying itself at the spot known as the origin of replication. Once two origins are present, they begin to move to alternate sides of the cell, each pulling the chromosome along with them.
While this is happening, the cell is expanding to a size much larger than it normally would be. This elongating process helps to separate the newly forming DNA replications. Once replication is complete, and the two sets of DNA have cleared the middle of the cell, then a process of cytokinesis occurs wherein the cell pinches inward to regenerate a new wall, or septum, which divides the two DNA copies.
This is all regulated in part by the septal ring–a ring of proteins formed near the middle of the cell–which encourages the cell to split evenly without causing damage to the cell wall or DNA, respectively. (We will take a further look into some of the specific proteins and their purposes later in this article).
This process is more successful when the organism is within an environment that is stable enough to support this rapid duplication. Ideally, a parent cell divides into daughter cells that contain the same DNA molecule as the original, and therefore are fully capable of all the same functions; Each cell becomes an independent organism.
Errors in the fission process can cause extra copies of genes or missing DNA within the daughter cells. This may leave the modified daughter cells susceptible to death if it is not stable enough to survive. Or it may be beneficial, as these fission errors are how genetic diversity is introduced. When this happens to cells preparing for mitosis, the errors in replication can lead to cancers or other serious conditions in the larger organism these cells occupy.
This is a quick video that will reinforce and summarize everything you just read along with some helpful images in just one minute:
This Sounds A Lot Like Mitosis… What’s The Difference?
Binary fission and mitosis are both processes of asexual reproduction wherein a parent cell splits to form two identical daughter cells. Most notably, we notice where each of these types of cell division processes occur. Binary fission primarily occurs in prokaryotes (bacteria), whereas mitosis happens eukaryotes (plant and animal cells).
Binary fission is much simpler than mitosis because it does not involve complex chromosomes, so the process can occur much faster. Specifically, mitosis requires the formation of the spindle apparatus–a bundle of fibers that act like the cell’s skeleton, in a way. It collects and organizes the chromosomes during mitosis. This helps to make sure the genetic material is distributed evenly; however, this process takes time.
Furthermore, mitosis is a means in growth, repair, and development, not just reproduction. Binary fission is simply for reproduction. The table below gives a side by side comparison of these two processes so you can more easily see the similarities and differences between them.
| Binary Fission | Mitosis |
|---|---|
| Asexual reproduction in which one cell divides to form two individual cells each containing the same genetic material. | Asexual reproduction of cells within the larger cell cycle. Usually occurs in parts of complex organisms. |
| Occurs primarily in prokaryotes (bacteria) | Occurs in eukaryotes (plants and animals) |
| Functions primarily as the reproduction process | Functions include repair and growth as well as reproduction |
| Process is simple and occurs quickly–replication and separation happen at the same time | Process is complex and occurs in phases which takes longer–replication is completed before cell division starts |
| No spindle apparatus assembled | Spindle apparatus is formed |
| Susceptible to unstable environments | Can occur in less favorable environments |
| Errors in replication are more common | Phases allow for checkpoints in “quality control” resulting in far fewer errors |
| Parental identity is preserved | Parental identity is preserved |
A Deeper Look At Binary Fission
As we’ve discussed, bacteria and archaea replicate via binary fission, but we also see this asexual reproduction method used in organelles. The general process and result remain the same in that the cell will divide into independent organisms, but there are some specific details we’ve observed among the various organisms that offer us some insight into this process.
Binary Fission In Bacteria
Since bacteria have simple genomes, binary fission is a stable process that results in relatively few mutations compared to eukaryotic replication. However, certain kinds of bacteria have developed a variation in the process.
Bacillus subtilis, for example, is a bacteria that lives in the soil as well as the gut of humans and some other mammals. It can divide equally to produce two similar cells, but it can also make smaller divisions that behave more like spores. These endospores often exhibit more resilience to harsh environments, i.e., they appear to be an evolution of the original organism.
There has also been a noticeable difference in how some bacteria lengthen and grow before they divide. Observations include extensions from the middle as well as the far ends of the cell. Also, it appears as though the time it takes for bacteria to divide varies among species and is controlled by their genetics. Some bacteria can fully split in less than 30 minutes, while others may take hours.
Binary Fission In Organelles
Mitosis is a more complex process than binary fission because it requires eukaryotes to duplicate larger genomes and many organelles, which require their own processes, therefore giving us the multiple phased steps of mitosis. And one of those processes within those phases includes the organelles replicating via binary fission.
A large number of organelles hold on to their own DNA, as this will guide them in their functions and support proper growth. The most well-known organelle example, mitochondria, needs to replicate itself into multiple copies in order to give a dividing cell the energy it needs to successfully divide. Through binary fission, the mitochondria are capable of obtaining the large numbers it needs quickly so the mitosis process can continue.
Throughout the eukaryotic cell, each organelle has to be copied at least once so as to provide the correct number of organelles in the developing cells. At the same time, the organelles are undergoing binary fission. They are also being pulled by the spindle apparatus and microtubules towards alternate sides of the cell wall. This way, each cell is set and ready for cytokinesis in which afterward, they have an immediate ability to function independently.
We will see this mitochondrial fission occurring quite often in the cell, even if it is not going through mitosis at the time. This is because its replication is used to create energy that will regulate the cell’s metabolism. Again, we see here the benefit of and necessity for the efficient and quick process of binary fission.
Binary Fission In Archaea (Part I)
As it was suggested in a previous statement, archaea exist as a sort of middle ground in the evolutionary link between prokaryotic bacteria and the more complex eukaryotic organisms. For this reason, it has been determined that a single explanation for the binary fission process in these organisms does not exist.
Rather there are several methods or machinery that may be utilized within the cell for cell division. Before continuing, it is important to know about some of the genetic machinery required for binary fission to occur. Let’s take a look at a few specifics.
The ESCRT Mechanism And Its Role In Binary Fission
Endosomal Sorting Complexes Required for Transport (ESCRT) machinery is composed of various protein complexes and categorized as ESCRT-0, ESCRT-I, ESCRT-II, and ESCRT-III. These complexes have been isolated, and numerous studies have been conducted for these organisms existing primarily in yeast and in humans. It has been found that ESCRT machinery plays a very important part in many cellular processes.
- Multivesicular Body (MVB) Biogenesis – MVB biogenesis is a process where ubiquitin proteins travel into specific organelles to destroy any damaged proteins. ESCRT machinery works to control this, and without it, the damaged proteins would build up and lead to disease.
- Cellular Abscission – Cellular abscission or cytokinetic abscission refers to the separation of the membrane connecting two daughter cells in binary fission. Without the ESCRT complexes, this division would not be possible, and replication would result in cells with twice the amount of DNA, which would eventually be destroyed through apoptosis (cell death).
- Viral Budding – Viral budding is a way that particular kinds of viruses exit cells. It is believed that ESCRT machinery also makes this possible, and ergo in their absence, the spreading of diseases from cell to cell would not be possible.
The FtsZ Protein
FtsZ is a protein that is encoded by the FtsZ gene. Its initials stand for “Filamenting temperature-sensitive mutant Z.” This protein is to prokaryotes what the tubulin protein is to eukaryotes, or to be more precise, FtsZ has been determined to be a homolog to tubulin. It exists in almost all bacteria and archaea and also in chloroplasts and some mitochondria where it is needed for cell division.
In particular, FtsZ is the protein that arrives first at the point of division between two cell copies and puts together the skeletal support for the Z-ring. FtsZ “recruits” other proteins that create the septum, which is what constricts to cause a divide in the cell. Conclusively, FtsZ is essential to the cytokinetic process.
Binary Fission In Archaea (Part II)
A particular classification of archaea known as crenarchaeota does not have a cell wall needed for mitosis, nor do they contain the FtsZ mechanism. It was found that these organisms use a very primitive version of the ESCRT-III system for eukaryotes in order to coerce the membrane into division by presenting itself in between the emerging daughter cells.
Recently, a new system of cell division was discovered in a specific crenarchaeota that is made up of homologs of eukaryotic ESCRT-III proteins. Based on that discovery, an analysis comparing the genetic machinery needed for cell division was done that uncovered at least three separate membrane remodeling systems may exist in archaea:
- The FtsZ-based bacterial-type system
- The ESCRT-III based eukaryotic-like system
- A unique system that uses an archaeal actin-related protein
Many genomes of archaea are capable of encoding a variety of parts from different complex systems. From this, it was suggested that the last common ancestor to today’s surviving archaea perhaps had complex membrane remodeling machinery that eventually lost through continuous evolution. The divergence can be seen in that eukaryotes appear to have inherited all three of these ancestral systems.
The Evolution Of Binary Fission
While it is determined that FtsZ plays the same part in cell division as tubulin does in eukaryotic cell division, there does not appear to be any kind of motor protein associated with FtsZ comparable to the actomyosin ring in eukaryotes. Overall, the question as to where the cytokinetic force originates has yet to be fully explained; however, there have been some studies that have made some interesting discoveries.
A recent study of Z-ring contraction observed that the rate at which a division in the cell occurred was affected primarily by a synthesis enzyme that slowed this process. It suggests that cell wall synthesis and chromosome segregation perhaps contribute to the division of the cell, and thereby challenges the theory that the Z-ring provides the driving force for cytokinesis.
Experiments were also done where a constructed membraned-targeted FtsZ was capable of producing visible constrictions in liposomes, suggesting that FtsZ can assemble the Z ring and generate a force on its own without the need for other proteins.
Furthermore, it has been proposed that the roles that proteins like tubulin and actin play in cell division have been reversed in the grander-timeline of evolution. It was identified in a new structure-based sequence alignment that amino acids in tubulins are conserved from FtsZ.
The observation of the use of FtsZ ring for the division of chloroplasts and in some mitochondria contributes to the evidence that they have prokaryotic ancestry. We know that organelles in eukaryotic cells used binary fission to reproduce, and it has determined that all chloroplasts and some mitochondria (in plants) that are derived from endosymbiosis of bacteria use the FtsZ protein in a manner similar to how bacteria use it for binary fission. Additionally, it has been noted that L-form bacteria may have held onto various mechanics of an ancient method of cell division.
Ancient ESCRTs In The Evolution of Binary Fission
It is well known that these tubulin-like proteins have an important role in separating DNA and dividing cells. But it has been recently uncovered that cell division is possible without any highly conserved protein. Again we see more than one distinct machinery used for binary fission has evolved in prokaryotes. A system dependent on tubulin-like protein is present, as is the ESCRT system.
Conclusion
Binary fission is a process of asexual reproduction used commonly by unicellular organisms such as bacteria and archaea. Certain organelles in eukaryotic cells are also known to replicate via binary fission. It is a very common and efficient process of DNA replication, and advanced studies are finding out more about this process and how it has evolved as well as the organisms that utilize it.
References
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- Britannica: Binary Fission
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