Binary fission is one of the most fascinating biological processes, period! It’s the ultimate existential magic, one that baffles us sexually reproducing creatures, leaving us wondering how the world it’s done.
Well, my friends, that’s exactly what I’m going to be talking about today.
We’ll be discussing what binary fission is, the different kinds, how archaea and bacteria pull off such a miraculous feat, and how the process diverges from typical mitosis.
Let’s dive right in!
Binary Fission: A Definition
A form of asexual reproduction, binary fission is a process by which certain domain members of the bacteria and archaea family split into two discrete organisms via cell division.
In simplified terms, binary fission is conceptually akin to mitosis, a process in which, again, one organism splits into two identical organisms, thereby eliminating the need for an external partner to reproduce.
That said, the two processes are in fact quite dissimilar in a few key ways.
For instance, mitosis is the result of the natural process of eukaryotic cells, whereas binary fission occurs as a means of propagation within the orders mentioned above.
Furthermore, the triggers for mitosis and binary fission are different, but the result is, of course, similar: the splitting of DNA, and eventually, the creation of daughter cells.
Let’s discuss the differences between these similar processes in greater detail.
Binary Fission Vs Mitosis: The Same But Different
Binary fission is a way for very simple organisms such as bacteria to multiply. Eukaryotic cells, the ones that engage in mitosis, by contrast, are far more complex structures.
In essence, binary fission is the most rudimentary form of reproduction, which is why it suits these most rudimentary organisms: bacteria and archaea. The process goes a little something like this…
- A single DNA molecule of the prokaryotes readies itself. This is an individual chromosome.
- The chromosome begins to uncoil.
- An exact copy of the chromosome is reproduced.
- The chromosomes, still within a single prokaryotic cell, then start to diverge.
- The cell begins stretching length ways.
- While this is happening, the prerequisites for cellular survival, otherwise known as cell organelles, are produced en masse, ensuring that when the cell splits, there are enough essential building blocks to share. These organelles include but are not limited to ribosomes and plasmids.
- Upon reaching the corresponding poles of the cell, the cell starts dividing.
- Finally, the cell splits in two!
This is, however, a very basic overview of the process. A more scientific explanation of the phases of binary fission is as follows.
- Phase 1. Chromosome replication
The genetic code of the chromosome is doubled, meaning the single cell is now essentially two cells in one.
- Phase 2. Cellular growth
In the second phase of binary fission, the cell starts to expand, which is essential to the eventual splitting further down the timeline. This stage occurs in tandem with the increased production of organelles.
The migration of the chromosomes to the poles of the cell is also considered part of this stage.
At this point, the chromosomes are considered “strands”, as they are technically still connected. The migration of the strands consumes a significant amount of energy within the cell.
- Phase 3. Chromosome segregation
Here, the chromosomes have made it to their respective peripheries, and a septum forms in the central space they once inhabited, thereby splitting them once and for all, albeit within the same cell (for now).
- Phase 4. The splitting of the cell
The cell continues to elongate until it the septum is peeled in two, leaving two daughter cells in place of the single parent cell.
In effect, what happens here is the septum becomes the fresh cell wall for both new cells.
The increase in organelle production means each daughter cell gets everything it needs to thrive. However, this is not always a perfect process.
As the way of the world, biological processes can be quite delicate, and any number of things can go wrong during one or more of the phases. This can lead to a variety of abnormalities.
It’s also worth noting that certain organelles can engage in binary fission themselves as a means of increasing in volume.
One example of this being the case is with mitochondria organelles.
In a nutshell, this is how binary fission goes down, but wait! We’ve yet to talk about the different types of binary fission, of which there are four:
- Irregular binary fission
- Longitudinal binary fission
- Oblique binary fission
- Transverse binary fission
But perhaps I’m getting ahead of myself here. Let’s save that for later and return to the matter at hand… binary fission vs mitosis.
As mentioned earlier, mitosis occurs in much more complex eukaryotic cells, an example of which is a human cell.
These cells feature organelles ensconced by cytoplasm, cytoskeletons, and, perhaps most importantly of all, a nucleus cushioned within a nuclear envelope.
During mitosis, the genome of a cell is duplicated to create the potential for two daughter nuclei.
Much like in binary fission, these two pieces of identical genetic code head for the poles of the cell, eventually causing a rift in the cytoplasm, followed swiftly by the division of the cell.
Mitosis Phase By Phase
Taking place in a more complex cell, it should come as no surprise that mitosis is a more complex process than binary fission.
In light of this, there are more mitotic steps to consider than there are steps in binary fission. It all begins with something known as interphase.
While not technically an active phase of the cell cycle, interphase is still essential. It’s composed of small preparatory phases that, when complete, set the stage for mitosis to start in earnest. Let’s take a closer look at these mini stages.
The G1 stage is very similar to the second phase of binary fission. The cell starts to grow, and as it expands, organelle production is kicked into hyper drive, alongside the production of all the other essentials of mitosis.
S phase is defined by the creation of a DNA molecule within the nucleus of the eukaryotic cell. What’s more, it is characterized by the multiplication of something known as a centrosome, a microtubule unit instrumental in splitting the genetic information when mitosis is in full swing.
The final stage of interphase is known as G2, and it’s relatively similar to G1, although in this stage, cellular growth is accompanied by perpetual reorganization of cell material.
This shift in structure occurs to facilitate smooth mitosis.
And thus, interphase terminates, making way for M phase, and yes, the M does indeed stand for mitotic. This stage is characterized by the duplication of DNA, which will eventually be used to form the basis of the daughter cells.
M phase is broken down into two smaller sections. The first is mitosis, and the second is cytokinesis. Then, these smaller stages are themselves broken down into even more specific processes.
The mitotic phase begins with prophase.
Well, to be more precise, it begins with pre-prophase, in which the nucleus makes a beeline for the middle of the cell. At one point in prophase, the nucleolus vanishes, setting the foundation for the development of spindle fibers.
A number of other events take place in prophase, including:
- The tight coiling of the chromosomes
- Centrosomes appear in close proximity to the nucleus.
- The centrosomes become surrounded by fibrous protein.
After prophase comes metaphase.
In this second phase of mitosis, we see a lot more development. For example…
- We see the nuclear envelope fade away.
- The spindle starts to develop.
- The chromosomes are yanked to the poles of the cell by microtubule contractions.
- The equatorial plate of the cell provides a stage for chromosomal alignment.
Next in sequence, we have anaphase.
This is the centrosomes’ time to shine. They’ve already been pretty involved via the contracting of microtubules, small structures located on the centrosomes, but now they really kick it up a notch.
They contract the microtubules even more ferociously, which has three effects. Firstly, the chromatids are severed, forming two discrete daughter chromosomes.
Secondly, these chromosomes are forced into opposite poles of the cell. And thirdly, a little later on, the pressure of these contractions aids in expanding the cell.
Then comes the telophase.
Here we have the final mitotic stage, and it involves further cell elongation thanks to the loosening of the microtubules and the development of new nuclear envelopes — one for each chromosome.
During this chromosomal swaddling process, the chromosomes become increasingly dense as they develop into their own nuclei.
The slow separation of the nucleus that transpires over the course of these four preparatory phases is known in scientific terms as karyokinesis.
Once these processes are complete, we reach the cytokinesis stage.
Cytokinesis — Simply put, cytokinesis is all about making good on the preparation that has already happened in the first half of M phase. The big headliner in this phase is the division of the cytoplasm, thereby creating the two daughter cells.
This phase doesn’t wait for M phase to terminate, rather, the two phases are blurred into one another. As M phase is wrapping things up with telophase, cytokinesis hits the ground running.
The mechanisms involved in the splitting of the cell are a contractile loop and a cleavage furrow.
Mitosis: Type By Type
Much like there are different forms of binary fission, there are different types of mitosis; however, there are only two types of mitosis as opposed to binary fission’s four.
These two types of mitosis are distinguished by the behavior of the nuclear envelope within the cell.
- Closed mitosis (or closed division)
Remember earlier when I mentioned that the nuclear envelope vanishes, and later on, two new ones form around the separated nuclei? Well, this isn’t always the case.
In closed mitosis, the nuclear membrane never dissipates. Instead, the microtubules of the centrosomes do the split right there within the membrane, an occurrence that often takes place in fungi, algae, and other such organisms.
- Open mitosis
Open mitosis is the form we’ve already discussed. Here, the bi-layer nuclear envelope does dissipate, making it extra easy for the microtubules to cleave the chromosome. This process is much more prevalent in multicellular organisms.
Side Note — It’s worth mentioning that certain organisms, such as rhizaria, fungi, and amoebozoa will exhibit both closed and open mitosis.
Types Of Binary Fission
Right, let’s get back to what I was saying earlier about the different types of binary fission. As established, there are four different types. Each of them is distinguished by the location of the final division.
- Irregular binary fission
Irregular binary fission refers to cell division in which the cytokinesis (the division itself) occurs perpendicularly to the karyokinesis. In other words, in any instance that the plane of division is perpendicular to the plane of karyokinesis, it’s considered irregular binary fission.
- Longitudinal binary fission
If the fission takes place on the longitudinal axis of the parent cell, then it’s considered an example of, you’ve guessed it… longitudinal binary fission.
- Oblique binary fission
If fission occurs in a nondescript fashion, with no notable relationship to any other zone or event, then it’s lumped under the umbrella term of oblique binary fission.
- Transverse binary fission
Transverse binary fission is the axial opposite of longitudinal fission, in that it occurs on the transverse axis rather than the longitudinal one.
To give you a better visual understanding of these types, picture da Vinci’s Vitruvian Man. The longitudinal line would be vertical, traveling from the man’s head to his feet, while the transverse line would be horizontal, traveling across the man’s midriff.
How Does Binary Fission Differ From Mitosis?
We’ve established that binary fission is fundamentally different from mitosis in that they occur in completely different cells, but that’s a very macroscopic view of these inherently intricate processes.
So, let’s take a closer look and detail some of the lesser known deep-cut differences.
- The spindle apparatus that develop during mitosis to aid in chromosome separation do not come to fruition in binary fission. They’re just not part of the process.
- As much more complicated biological structures, eukaryotes contain a lot of organelles that multiply in preparation for fission, but only a few organelle levels are increased in a prokaryotic cell before binary fission occurs. One such organelle is the ribosome.
- Binary fission in prokaryotes is a means of reproduction exclusively — that’s it. Mitosis, on the other hand, has multiple functions. It’s instrumental during organism progression, cell growth, and cell replacement.
- Being that mitosis is a much more complex procedure taking place in more sophisticated structures, it tends to be quite a lengthy process. Binary fission, on the other hand, can happen remarkably quickly.
How Does Binary Fission Work In Single-Celled Organisms?
To give you an idea of what this process would entail in a single-celled organism, let’s use a specific example.
Paramecium And Binary Fission
Although the term paramecium refers to single-celled organisms, it doesn’t refer to a single organism. It’s actually a genus containing multiple ciliates, including paramecium bursaria, paramecium aurelia, and paramecium caudatum, to name just a few.
This family of single-celled organisms have a very distinct appearance. They look reminiscent of the sole of a shoe, and they’re coated in cilia — microscopic, hairlike tendrils that provide either propulsion through or turbulence in surrounding fluid.
While technically quite complex eukaryotic cells and perfectly able to reproduce sexually, asexual reproduction is the optimal means of propagation for these tiny organisms. In ideal conditions, this is how they will choose to operate.
In the case of paramecium, transverse binary fission takes place, and although there are already two nuclei in this kind of cell, only the smaller of the two does the heavy lifting at first.
In conditions that allow for this form of reproduction (which is usually water), the oral passage used for feeding shuts down, and the micronucleus (the smaller one we just talked about) gets to work, eventually splitting into two discrete micro nuclei.
Then, it’s par the course, with the nuclei heading for the poles of the cell, but at this stage, something interesting occurs.
The larger nucleus then follows suit and begins this cellular cloning process. Once split, the two nuclei head to their corresponding poles, setting the stage for cytokinesis on the transverse axis.
Although the micronucleus does utilize mitotic apparatus during asexual reproduction, the greater of the nucliei does not use any such apparatus (amitosis), which is why this form of propagation is considered transverse binary fission.
A more detailed explanation would be that the actual cell growth of the second cell does not occur when the micro nucleus is in action. At this stage, all available energy is spent on the duplication process rather than the formation of the daughter cells.
The cell growth and subsequent complete cytokinesis only takes place once the larger of the nuclei divides and reaches the poles of the cell, and as this division uses no mitotic apparatus, the split is the product of binary fission and not mitosis.
How Does Binary Fission Work In Bacteria?
Before we dig in here, it’s important to note that bacteria have numerous means of reproducing asexually. Depending on the conditions, they might engage in:
- Binary fission
- Cindia/gonidia formation
- Zoogloea stage
Lots of bacterial life forms can try their hand at these reproductive options, but, like paramecium, they much prefer to reproduce using binary fission, should conditions allow it, of course.
Sensitive environmental conditions include whether they’re in water or not and the temperature of said water.
If they are in water, the temperature is just right, and all other sensitive parameters are in the Goldilocks zone, bacteria will follow two primary phases of binary fission.
By this point in the guide, these phases will be nothing new to you.
- Genome duplication
Before a cell can split in two, it needs to harbor two sets of genetic information, so to kick things off, enzymes in the bacteria start replicating the chromosome.
They begin at the origins, creating a chromosome strand, and once ready, will cleave the strand.
These origins will then start their journey to the poles of the cell while the DNA is still being replicated.
- Formation of the septum followed by division
This phase involves the formation of the bi-membrane septum and the peripheral ring splitting in two. This division utilizes no mitosis apparatus whatsoever, and in optimal conditions, full binary fission can occur in roughly half an hour.
How Does Binary Fission Work In Amoebas?
Much like paramecium, amoebas are actually eukaryotes. They are ill-defined, visually speaking, and get around using false feet (pseudopodium), temporary limb-like protrusions.
They also use these pseudopodia to feed.
For the most part, a species like Amoeba Proteus will use binary fission as a means of reproduction, but if conditions are suitable, they may also engage in sporulation or even multiple fission.
Again, like paramecium, the actual duplication of the DNA occurs via mitotic principles, but cytokinesis itself is the product of irregular binary fission.
Unlike fission in bacteria, this form of asexual reproduction in amoeba does involve the creation of nuclear spindle.
That’s all the essentials covered, I believe. This has been a crash course on binary fission covering everything from the definition, to the process itself.
Hopefully the sheer volume of information laid on you today hasn’t left you with a… splitting headache (sorry, I couldn’t help myself).