Binary fission: characteristics and stages of this reproductive process

What is binary fission? Let's look at the characteristics of this type of bacterial reproduction.

Bacteria are all around us, even if we are not able to see them. These microorganisms are essential for life in all terrestrial ecosystems, as they are vital in biogeochemical processes such as the decomposition of organic matter, the realization of the nitrogen cycle, the production of oxygen (photosynthetic bacteria) and much more.

It is estimated that bacteria contribute 15% of the total terrestrial biomass (70 gigatons), second only to plants. In addition to being on all living surfaces, these living beings also live inside us: our colon contains 1014 bacterial units, which help us to break down plant matter, actively prevent infection by other microorganisms and enable the development of the immune system during our first steps as human beings.

All these facts and figures are exciting, but we don't want to stop there. To understand the importance of bacteria in the world, it is necessary to investigate their way of life, and what less than describing their reproduction to find out how bacterial colonies remain stable over time. Based on this very interesting premise, we tell you all about binary fission.

What is binary fission?

Binary fission is a type of asexual reproduction that takes place in bacteria and archaea, i.e. microscopic prokaryotic organisms.. Before continuing, we must establish a series of basics regarding reproduction.

We have said that we are dealing with a type of asexual reproduction, whose premise is basically the same as that of mitosis in multicellular organisms. Our somatic cells (of tissues) divide by this mechanism, that is, the division of a parental cell into two daughters with the same shape, size and genetic information. However, mitosis and fission present a series of very important differences.

Broadly speaking, it is essential to emphasize that mitosis is unique to organisms with more than one cell.. This mechanism of cell division is intended to increase or replace the cells of a tissue and is therefore used for the growth, development and repair of the organs that compose us. On the other hand, binary fission follows a much simpler premise: where there was once one bacterium, there are now two.

For this reason, binary fission is a type of asexual reproduction that is only conceivable in prokaryotic organisms, i.e. those composed of only one cell (bacteria and archaea, in this case). If it were observed in a multicellular organism, it would be a case of mitosis. It is as simple as that.

Steps of binary fission

Most bacteria reproduce by binary fission, as this mechanism causes an exponential increase in the number of this mechanism causes an exponential increase in the number of individuals in a colony.. Where there was once one microorganism, there are now two, then four, then eight, then 16, 32, 64, 128, etc. To give you an idea, the bacterium E. coli under optimal conditions can divide by fission once every 20 minutes. As you can imagine, in 24 hours the number of bacterial units is inconceivable with this reproductive rate.

The following is a brief description of each of the stages into which binary fission is divided. We are sure that many of the mechanisms described here are familiar to you, as they are very similar to those of mitosis. Let's get to it.

1. DNA replication

For a bacterium to be able to divide into two of its own, it must be able to self-replicate its genetic information.. Many of the microorganisms studied have only one circular chromosome in their nucleoid (unlike the 46 in the nucleus of human cells), so we will take this general rule as a reference.

The bacterial chromosome is inherently a replicon, as this term refers to a unit of genetic information that contains all the elements necessary to carry out the replication process. This conglomerate of DNA replicates at a single origin, which moves linearly until the total duplication of the entire molecule is completed.

We are not going to dwell on complex processes such as the structures involved, the replication fork and others. It is enough for us to know, in this case, that the enzymes that make this mechanism possible are known as DNA polymerases and that it is a semi-conservative process, i.e. each new molecule formed contains an old and a new strand of DNA..

Chromosome segregation

In normal mitosis, chromosomes are randomly placed at the cell's equator, waiting to be "pulled" by the mitotic spindle to each extreme pole of the cell body. In meiosis (which gives rise to gametes) this moment is really important, since chromosomal permutations at the cellular equator can result in thousands of different combinations as far as genetic distribution is concerned.

In this case, the matter is much less exciting, because we have only two chromosomes resulting from the replication of one chromosome.. The two chromosomes move and segregate to each pole of the bacterial cytoplasm, without further complication.

3. Separation

As each chromosome travels to one pole, the bacterial membrane invaginates to form a septum, also known as the division wall, within the cell.inside the cell. When the septum divides, both bacteria with the corresponding genetic information become individual entities capable of autonomous survival.

The evolutionary significance of binary fission

It should be noted that there are several types of binary fission depending on the plane of cleavage (regular, amoeboid, transverse, oblique, etc.), but we do not want to focus on technical terminology. In closing, we find it much more interesting to explore the why of this simple yet essential mechanism.

The key to bacterial binary fission can be encapsulated in a single concept: logarithmic release.. This term refers to the second phase of bacterial growth, after habituation of the microorganisms to the new medium into which they are introduced. During this stage, an exponential increase in the bacterial growth curve is observed, i.e. the more bacteria are found in the initial population, the more they will be able to divide.

It should be noted that the slope of the logarithmic function depends on the conditions of the medium, since it is not the same to grow in a warm and sheltered place as it is to grow in the North Pole. In any case, the stabilization of growth (passage to the stationary phase or "plateau") is conditioned by the availability of nutrients: bacteria stop dividing when there are no more means to survive.

This is a clear example of a "quantity over quality" strategy. All bacteria are genetically the same as the progenitor (since binary fission is a type of asexual reproduction), so their adaptive capacity is the same, isn't it? To understand the success of binary fission, we must also take into account that the mutation rate of the bacterial genome is very high.

Therefore, it is not always assured that a bacterial generation will be the same as the previous one, something tremendously beneficial for the adaptive capacity of these microorganisms. Mutations are random, so some can be bad and some good, but the key difference is that the good ones are fixed in the population, while the negative ones disappear.while the negative ones disappear.

Thus, the faster a bacterial population divides, the more likely it is that a mutation will appear that allows better adaptation to the environment. This is the basis for the existence of antibiotic-resistant microorganisms: binary fission and the growth of bacterial populations give them the ability to become resistant to even the most specific drugs.

Summary

As you have seen, everything in nature has an explanation, except in exceptional cases. Binary fission is a reproductive strategy just as valid as sexual reproduction for prokaryotic organisms, since they obtain the genetic variability necessary to adapt from mutations in their genome, and not through the union of a female and a male gamete (as occurs in our species).

At the end of the day, the whole evolutionary process can be summarized in the following sentence: living beings do what they can with what they have. The binary fission mechanism may not be perfect, but it has certainly allowed the permanence and expansion of these microorganisms on Earth for centuries.

Bibliographical references:

• Eswara, P. J., & Ramamurthi, K. S. (2017). Bacterial cell division: nonmodels poised to take the spotlight. Annual review of microbiology, 71, 393-411.
• Binary fission, Khan Academy. Recogido a 25 de marzo en https://es.khanacademy.org/science/biology/cellular-molecular-biology/mitosis/a/bacterial-binary-fission
• Margolin, W. (2014). Binary Fission in Bacteria. eLS.
• Nyström, T. (2007). A bacterial kind of aging. PLoS Genet, 3(12), e224.
• Samson, R. Y., & Bell, S. D. (2009). Ancient ESCRTs and the evolution of binary fission. Trends in microbiology, 17(11), 507-513.
• Smith, J. M., Smith, N. H., O'Rourke, M., & Spratt, B. G. (1993). How clonal are bacteria?. Proceedings of the National Academy of Sciences, 90(10), 4384-4388.