Binary Fission in Bacteria

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.

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.

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.

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).

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.

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.

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