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Binary Fission in Bacteria

Binary Fission in Bacteria
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Prokaryotes, such as bacteria, are the cause of many diseases that affect humans. We deal with them every day without even thinking about it. From washing our hands to disinfecting high-use areas such as doorknobs, desks and tables, and even our phones!

But you may wonder, how often do I really need to be washing my hands, or disinfecting surfaces? Can bacteria really reproduce that quickly? YES! Because prokaryotes, specifically bacteria, are simple compared to eukaryotes, they can reproduce much, much faster. Some bacteria can reproduce every 20 minutes! To put that in perspective, at that rate, a single bacterium can grow to a colony of 250,000 within 6 hours! How is that possible? Well, it's all thanks to a process called binary fission.

Binary Fission In Bacterial Cells

We have learned how eukaryotic cells divide through mitosis or meiosis. But cell division in prokaryotic cells is different. Most prokaryotic organisms, bacteria and archaea, divide and reproduce through binary fission. Binary fission is similar to the Cell Cycle because it is another process of cellular division, but the cell cycle only occurs in eukaryotic organisms. Just like the cell cycle, binary fission will begin with one parent cell, then replicate its DNA chromosome, and end with two genetically identical daughter cells. While the daughter cells are clones, they are also individual organisms because they are prokaryotes (single-cell individuals). This is another way binary fission differs from the cell cycle, which produces new cells (for growth, maintenance, and repair in multicellular eukaryotes) but no new individual organisms. Below we'll go further in-depth on the process of binary fission in bacteria.

Binary fission is a type of asexual reproduction in single-cell organisms where the cell doubles in size and separates into two organisms.

In protists, cell division is also equivalent to organism reproduction since they are single-cell organisms. Thus, some protists also divide and reproduce asexually through binary fission (they also have other types of asexual reproduction) in the sense that a parent cell/organism replicates its DNA and split into two daughter cells. However, protists are eukaryotes and therefore have linear chromosomes and a nucleus, consequently, binary fission is not the exact same process as in prokaryotes as it includes mitosis (it is a closed mitosis in most protists though).

Process of binary fission in bacteria

The process of binary fission in bacteria, and other prokaryotes, is much simpler than the cell cycle in eukaryotes. Prokaryotes have a single circular chromosome that is not enclosed in a nucleus, but is instead attached to the cell membrane at a single point and occupies a cell region called the nucleoid. Prokaryotes do not have histones or nucleosomes like eukaryotic chromosomes, but the nucleoid region contains packaging proteins, similar to condensin and cohesin, used in condensing eukaryotic chromosomes.

Nucleoid- the region of the prokaryotic cell that contains the single chromosome, plasmids, and packaging proteins.

Thus, binary fission in bacteria differs from mitosis because this singular chromosome and lack of a nucleus make the process of binary fission much simpler. There is no nucleus membrane to dissolve and dividing duplicated chromosomes does not require the same amount of cell structures (like the mitotic spindle) as in the mitotic phase of eukaryotes. Therefore, we can divide the binary fission process into only four steps.

Diagram of binary fission in bacteria

The four steps of binary fission are represented in Figure 1 below, which we explain in the next section.

Binary Fission Bacteria binary fission diagram StudySmarter

Figure 1: Binary fission in bacteria. Source: JWSchmidt, CC BY-SA 3.0 , via Wikimedia Commons

Steps of binary fission in bacteria

There are four steps to binary fission in bacteria: DNA replication, cell growth, genome segregation, and cytokinesis.

DNA replication. First, the bacteria must replicate its DNA. The circular DNA chromosome is attached to the cell membrane at one point, close to the origin, the site where DNA replication begins. From the origin of replication, the DNA is replicated in both directions until the two replicating strands meet and DNA replication is complete.

Cell growth. As the DNA is replicating, the bacterial cell is also growing. The chromosome is still attached to the cell's plasma membrane as it replicates. This means that as the cell grows it also helps to separate the replicating DNA chromosomes to opposite sides of the cell beginning genome segregation.

Genome segregation occurs continuously as the bacteria cell grows and the DNA chromosome replicates. As the chromosome is done replicating and has passed the midpoint of the growing cell, cytokinesis will begin. Now, remember bacteria also have smaller free-floating DNA packets called plasmids that are acquired from their environment. Plasmids are also replicated during DNA replication, but since they are not necessary for the function and survival of the bacteria cell, they are not attached to the plasma membrane and do not get distributed evenly across the daughter cells as cytokinesis begins. This means the two daughter cells may have some variation in the plasmids they possess, leading to variation in the population.

Cytokinesis in bacteria is almost a mixture of cytokinesis in animal and plant cells. Cytokinesis begins with the formation of an FtsZ protein ring. The FtsZ protein ring performs the role of the contractile ring in animal cells, creating a cleavage furrow. FtsZ aids in recruiting other proteins as well, and these proteins begin synthesizing new cell wall and plasma membrane. As the materials for the cell wall and plasma membrane accumulate, a structure called a septum forms. This septum is similar in function to the cell plate in plant cells during cytokinesis. The septum will fully form into new cell wall and plasma membrane, finally separating the daughter cells and completing cell division by binary fission in bacteria.

Some bacteria called coccus (that have a spherical shape) do not always complete cytokinesis and can stay attached forming chains. Figure 2 shows the bacteria Staphylococcus aureus, some individuals have undergone binary fission and the two daughter cells have not completed separation (the cleavage furrow is still visible).

Binary Fission Staphylocuccus aureus bacteria StudySmarter

Figure 2: Scanning electron micrograph of methicillin-resistant Staphylococcus aureus bacteria (yellow) and a dead human white blood cell (red). Source: NIH Image Gallery, Public domain, Flickr.com.

Examples of binary fission in bacteria

How long does binary fission in bacteria take? Some bacteria can reproduce really fast, like Escherichia coli. Under laboratory conditions, E. coli can reproduce every 20 minutes. Of course, laboratory conditions are considered optimal for bacterial growth as culture media have all the resources they need. This time (called generation time, growth rate, or doubling time) can differ in the natural environment where bacteria are found, either for free-living bacteria or the ones associated with a host.

Under natural conditions, resources can be scarce, there is competition and predation among individuals, and waste products in a colony also restrict bacterial growth. Let’s see some examples of doubling times (the time it takes for a bacterial colony in culture to double its number of cells) for normally harmless bacteria that can become pathogenic to humans:

Table 1: Examples of doubling times for bacteria under laboratory conditions and in their natural environments.

Bacteria

Natural habitat

Indirect estimation of doubling time (hours)

Doubling time in laboratory conditions (minutes)

Escherichia coli

Lower intestine of humans and free in the environment

15

19.8

Pseudomonas aeruginosa

Diverse environments including soil, water, plants, and animals

2.3

30

Salmonella enterica

Lower intestine of humans and reptiles, and free in the environment

25

30

Staphylococcus aureus

(Figure 2)

Animals, human skin and upper respiratory tract

1.87

24

Vibrio cholerae

Environments with brackish waters

1.1

39.6

Source: created with information from Beth Gibson et al., 2018.

As expected, it takes longer for bacteria to reproduce under natural conditions. It is important to note that the reproduction time in a laboratory culture probably corresponds to the time binary fission takes for a bacterial species, as they divide continuously under these conditions. On the other hand, bacteria are not dividing continuously in their natural environment, thus these rates mostly represent how often a bacterium reproduces.

Advantages of binary fission in bacteria

Binary fission, as a type of asexual reproduction, has some advantages such as:

1. It does not require the investment of resources to find a partner.

2. Rapid increases in population size in a relatively short time. The number of individuals that can reproduce doubles the number that would reproduce sexually (as each individual will produce offspring, instead of a pair of individuals).

3. Traits highly adapted to an environment are passed on without modifications (excluding mutations) to the clones.

4. Faster and simpler than mitosis. As described earlier, compared to mitosis in multicellular eukaryotes, there is no nucleus membrane to dissolve and complex structures like the mitotic spindle are not required.

On the other hand, the main disadvantage of asexual reproduction for any organism is the lack of genetic diversity among the offspring. However, since bacteria can divide so fast under certain conditions, their mutation rate is higher than for multicellular organisms, and mutations are the primary source of genetic diversity. In addition, bacteria have other ways to share genetic information among them.

The development of resistance to antibiotics in bacteria is a big concern currently as it results in hard-to-treat infections. Antibiotic resistance is not the result of binary fission, initially, it has to arise from a mutation. But because bacteria can reproduce so fast through binary fission, and as a type of asexual reproduction, all the descendants of one bacterium that develops antibiotic resistance will have the gene as well.

A bacterium without antibiotic resistance can also acquire it by conjugation (when two bacteria join to directly transfer DNA), transduction (when a virus transfers DNA segments from one bacterium to another), or transformation (when a bacteria take DNA up from the environment, like when released from a dead bacteria). As a result, a beneficial mutation like antibiotic resistance can spread real fast within a bacterial population and to other bacterial species.

Binary Fission in Bacteria - Key takeaways

    • Bacteria, and other prokaryotes, use cell division by binary fission to reproduce.
    • Prokaryotes are much simpler than eukaryotes and so binary fission can occur much more quickly.
    • Bacterial plasmids are also replicated during DNA replication but are haphazardly segregated into the two poles of the cell, thus chromosomes will be exact copies but there may be variation in the bacterial plasmids of the two daughter cells.
    • Compared to the mitotic phase of eukaryotes, there is no nucleus membrane to dissolve and a mitotic spindle is not required (the bacterial chromosomes are separated by the growing plasma membrane to which they are attached).
    • FtsZ proteins form a cleavage furrow and recruit other proteins to begin building the cell wall and plasma membrane, forming a septum in the middle of the cell.

References

Lisa Urry et al., Biology, 12th edition, 2021.

Mary Ann Clark et al., Biology 2e, Openstax web version 2022

Beth Gibson et al., The distribution of bacterial doubling times in the wild, The Royal Society Publishing, 2018. https://royalsocietypublishing.org/doi/full/10.1098/rspb.2018.0789

Image links

Figure 1: https://commons.wikimedia.org/wiki/File:Binary_fission.png

Figure 2: https://www.flickr.com/photos/nihgov/49234831117/

Frequently Asked Questions about Binary Fission in Bacteria

Binary fission is the asexual reproduction in bacteria where the cell grows in size and separates into two identical organisms.

The 3 main steps of binary fission in bacteria are: replication of the single circular chromosome, cell growth and segregation of the duplicated chromosomes to opposite sides of the cell (moved by the growing cell membrane to which they are attached), and cytokinesis through the formation of a contractile ring of protein and a septum that forms new cell membrane and wall. 

Binary fission occurs through the following steps in bacteria: replication of the single circular chromosome, cell growth, segregation of the duplicated chromosomes to opposite sides of the cell (moved by the growing cell membrane to which they are attached), and cytokinesis through the formation of a contractile ring of protein and a septum that forms new cell membrane and wall. 

Binary fission helps bacteria survive by allowing high reproduction rates. By reproducing asexually, bacteria do not spend time looking for a mate. Due to this and the relatively simple prokaryotic structure, binary fission can occur very fast. Although the daughter cells are typically identical to the parent cell, the high reproduction rate also increases the rate of mutations that can aid to gain genetic diversity.

Bacteria reproduce by binary fission through the following steps: replication of the single circular chromosome, cell growth, segregation of the duplicated chromosomes to opposite sides of the cell (moved by the growing cell membrane to which they are attached), and cytokinesis through the formation of a contractile ring of protein and a septum that forms new cell membrane and wall. 

Final Binary Fission in Bacteria Quiz

Binary Fission in Bacteria Quiz - Teste dein Wissen

Question

What is the equivalent to binary fission in eukaryotes?

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Answer

The Cell Cycle

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Question

What are the four steps to binary fission?

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Answer

DNA replication, Cell Growth, Genome Segregation, Cytokinesis

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Question

How is cytokinesis during binary fission similar to cytokinesis in an animal cell?

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Answer

During binary fission, a ring of FtsZ proteins forms a contractile ring and cleavage furrow. This is similar to the contractile ring of actin filaments and cleavage furrow in animal cells.

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Question

How is cytokinesis during binary fission similar to cytokinesis in a plant cell?


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Answer

FtsZ proteins trigger the synthesis of a new cell wall and plasma membrane, creating a septum. This is similar to the Golgi vesicles building a cell plate during plant cytokinesis.

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Question

What is the origin, in regards to binary fission?

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Answer

The origin is the site where DNA replication begins. It is near where the prokaryotic chromosome is attached to the plasma membrane.

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Question

How does the prokaryotic chromosome get segregated?

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Answer

As the cell grows, the attachment point of the duplicating chromosomes get separated to opposite sides of the cell, taking chromosomes with it.

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Question

How are plasmids segregated during binary fission?

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Answer

The duplicated plasmids are haphazardly distributed during genome segregation, leading to variation in the daughter cells.

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Question

Compared to the cell cycle, how quickly does binary fission take place?

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Answer

Binary fission is much faster than the cell cycle. It can be as quick as 20 minutes between divisions.

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Question

True or false: As in the cell cycle, binary fission results in two daughter cells that are part of the same organism.

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Answer

False. The result of binary fission is two daughter cells but these cells are individuals, not part of the same organism.

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Question

True or false: Prokaryotes have a chromosome and plasmids contained in a region called the nucleoid.

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Answer

True. Prokaryotic genomes are smaller and do not have a nuclear membrane to keep them contained.

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Question

Bacterial binary fission is faster than the eukaryotic cell cycle because: 

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Answer

bacteria do not replicate their DNA

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Question

Which of the following statements are true about binary fission?


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Answer

It is a type of sexual reproduction

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Question

The time it takes a bacterium to reproduce under laboratory conditions (optimal conditions) probably represents how often they reproduce in natural conditions.


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Answer

False

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Question

Why does it take longer for a colony of bacteria to double its population size in its natural habitat compared to laboratory conditions?


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Answer

Under natural conditions, resources can be scarce, there is competition and predation among individuals, and waste products in a colony also restrict bacterial growth. 

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Question

The ___ is responsible for splitting the parent cell in two, while the ___ forms a new cell wall. 


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Answer

FtsZ protein ring, septum

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