Binary Fission

Binary Fission Definition

Binary fission is the process through which asexual reproduction happens in bacteria. During binary fission, a single organism becomes two independent organisms. Binary fission is also used to describe the duplication of organelles in eukaryotic species, and is sometimes used to describe the reproduction of some invertebrates that asexually reproduce through budding. Though their cells undergo mitosis, the process is known as binary fission as it produces two organisms from one. In the similar multiple fission, an organism divides into more than two copies.

Binary Fission Steps

Binary Fission

Before binary fission of a prokaryote, as seen in step 1 of the above graphic, a prokaryote’s DNA is tightly wound. Sometimes, the prokaryote will carry small plasmids, which are small rings of DNA that carry extra genetic information. During the second step of binary fission, the DNA is unraveled. As it is unraveled, proteins gain access to the DNA, which are able to replicate the ring of DNA. The same proteins work on the plasmids in the cell, duplicating them as well. By step 3, both the DNA and plasmids have been duplicated. The individual copies of DNA attach themselves to different parts of the cell membrane. As the cell elongates in preparation for division, the DNA molecules are pulled to different sides of the cell.

At step 4, a cleavage furrow appears in the cell membrane, as the cell wall and membrane start to pinch off and create two new cells. Finally, as seen in step 5, the cells become completely separated from one another as a new bacterial cell wall forms. The final step includes breaking any additional proteins or other molecules that still connect the two cells. Each cell now has everything it needs to continue the functions of life independently.

Examples of Binary Fission

Binary Fission in Bacteria

All of the organisms in the domains Archaea and Bacteria reproduce asexually through binary fission. By far, bacteria account for the most populous organisms on the planet. The process of binary fission is a very stable one, and because bacteria have a very simple genome, there are relatively few mutations in prokaryotes as compared to eukaryotes. Eukaryotes must undergo many cell divisions before gametes can be produced for sexual reproduction, therefore many more mutations can be introduced before offspring are created.

Bacteria will go through the steps listed above as they proceed through binary fission. However, there are many alterations on this scheme that have evolved in the different lines of bacteria. For instance the bacteria Bacillus subtilis is a bacteria that exists in the soil and in the gut of some mammals, including humans. This bacteria can divide equally, creating two relatively identical cells, or it can create a much smaller division, which acts as a spore. This endospore is much more resilient than its larger counterpart, and can travel through an animal or the environment to new locations or simply survive until favorable conditions return. Bacteria also vary in the way that they elongate to divide. Some bacteria extend at the far end, while others grow from the middle outward. Even the timing with which bacteria divide is different, and directed by genetics. Some bacteria can divide in as little as 20 minutes, while other take many hours.

Binary Fission in Organelles

Although the process of mitosis in eukaryotes is similar to binary fission, it is much more complex because eukaryotes have larger genomes and many organelles to duplicate. However, the organelles of eukaryotes replicate using binary fission. Many organelles even harbor their own DNA, which directs their functions and growth. Mitochondria for example, the energy center of the cell, must make many copies of itself to provide a dividing cell with enough energy. Mitochondrial DNA is replicated, and the organelle divides in the same sequence described above.

Throughout the cell, each organelle must be replicated at least once, if the resulting cells are to have the proper amount of organelles. As the organelles undergo binary fission, they are also moved by the directions of the spindle apparatus and microtubules to opposite ends of the cells. Thus, when the cell divides through cytokinesis after mitosis, each cell is ready to operate independently immediately.

  • MitosisCell division in eukaryotes that produces two nearly identical daughter cells.
  • Meiosis – A method of cell division in eukaryotes in which the new daughter cells have less genetic material than the parent.
  • Multiple Fission – A single cell divides into more than two new daughter cells.
  • Plasmotomy – A multinucleated cell divides into two multinucleated cells.


1. Certain species of flatworms undergo a process known as fission, in which one worm splits into two. Flat worms are a eukaryotic, multi-cellular species. What type of cell division are the cells undergoing when the worm divides?
A. Meiosis
B. Binary Fission
C. Mitosis

Answer to Question #1
C is correct. While the process as a whole is known as binary fission, it does not mean that the cells are dividing the same way that bacteria divide. Eukaryotic cells must undergo the much more complex process of mitosis to divide. However, “fission” simply means the splitting of one entity into multiple entities. If two worms are produced, the act can be called “binary fission”, while the individual cells each undergo mitosis. If the flat worm can reproduce sexually, it will produce special cells for that process through meiosis.

2. A scientist is testing chemicals on bacteria, to try to figure out what they do. The scientist finds a substance that causes only one of the two daughter cells made during binary fission to be viable. When the scientist stains the cells to look for DNA, he finds no DNA in the dead daughter cell. Which of the following is NOT a possible effect of the unknown substance?
A. The substance caused the proteins that replicate DNA to malfunction, halting DNA replication.
B. The substance destroys DNA, and that is why the second cell is dead.
C. The substance causes microtubules to form incorrectly, keeping both copies of DNA in one cell.

Answer to Question #2
B is correct. Both A and C are possible. If the substance affected DNA replication, the cell would not be able to distribute copies of the DNA to each cell. To distribute the copies of DNA, the cells uses microtubules to attach the DNA to sections of the cell membrane. If these microtubules do not form correctly, all of the DNA could be left in one cell. The second cell would quickly die without genetic material to produce new proteins from. The scientist could tell if this had happened by comparing the stained cells to cells that only have one copy of DNA. If the cell is darker than cells with unduplicated DNA, it is a sign that multiple copies are present. The only option that cannot work is B. If the substance destroyed DNA, both cells would be affected as they are in the same environment.

3. A scientist is trying to measure the rate at which certain organisms undergo binary fission. The scientist takes a colony of bacteria from the soil and a colony of bacteria from a pond. He puts both in weak aqueous solutions, and measures the rate at which they increase. The bacteria from the pond multiply much faster in the condition, and the scientist declares them the species that undergo binary fission more often. Why is this a bad test of reproductive rate?
A. The soil bacteria are at a disadvantage, because they have not evolved for weak aqueous solutions.
B. One colony of bacteria got oxygen, while the other did not.
C. With so many bacteria in a colony, it is impossible to count them.

Answer to Question #3
A is correct. The soil bacteria are used to an entirely different set of condition than the pond bacteria. By conducting the experiment in a weak aqueous solution, the pond bacteria will be more adapted to the situation. The soil bacteria may have trouble getting nutrients or controlling the amount of water in their cells. This would slow the rate at which they can reproduce. A better experiment would test both colonies in the conditions they have evolved for, in a neutral condition, and in the opposite condition in which they exist naturally. This would give the scientist a better idea of how fast the bacteria can divide when they have all the nutrients they need, as well as when conditions are unfavorable.
  • 1
  • 1
Scroll Up