Binary fission: characteristics and phases of this reproduction process

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

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

All these figures and data are exciting, but we do not want to stay there. To know the importance of bacteria in the world, it is necessary to investigate their way of life, and what less than describe their reproduction to find out how bacterial colonies remain stable over time. Based on this very interesting premise, we will tell you everything about

binary fission.

• Related article: "Prokaryotic cells: what are they and what are their characteristics"

What is binary fission?

binary fission is a type of asexual reproduction that takes place in bacteria and archaea, that is, microscopic prokaryotic organisms. Before continuing, we must establish a series of bases as far as reproduction is concerned.

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 (tissue) cells divide by this mechanism, that is, the division of a parental cell into two daughters with the same shape, size, and genetic information. In any case, 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, therefore, it is used for the growth, development and repair of the organs that make up. On the other hand, binary fission follows a much simpler premise: where once there was one bacterium, now there are two.

For this reason, binary fission is a type of asexual reproduction that is only conceived in organisms. prokaryotes, that is, those that are only made up of one cell (bacteria and archaea, in this case). If it were observed in a multicellular organism, we would be facing a case of mitosis. Simple as that.

Steps of binary fission

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

Next, we briefly present each of the stages into which binary fission is divided. Surely many of the mechanisms collected here are familiar to you, since they are very similar to those of mitosis. Go for it.

1. Replication of DNA

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

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

We are not going to stop at 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, that is, each new molecule formed contains one old and one new DNA strand..

2. chromosome segregation

In normal mitosis, chromosomes are positioned at the equator of the cell in a random fashion, 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 truly important, since chromosome permutations at the cell equator can result in thousands of different combinations as far as genetic distribution is concerned. refers.

In this case, things are much less exciting, since we only have two chromosomes produced by the replication of one. The two chromosomes move and segregate to each pole of the bacterium's cytoplasm, without further complication.

3. Separation

As each chromosome travels to a pole, the bacterial membrane invaginates to form a septum, also known as the dividing wall., 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 is necessary to emphasize that there are several types of binary fission depending on the plane of division (regular, amoeboid, transverse, oblique, etc.), but we do not want to focus on technical terminology. By way of closing, we find it much more interesting to explore the reason for this mechanism, as simple as it is essential.

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

It should be noted that the slope of the logarithmic function depends on the environmental conditions, since it is not the same to grow in a warm and secluded place than to grow at the North Pole. In any case, the stabilization of growth (passage to the stationary phase or "plateau") is seen 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 identical to the parent. (because binary fission is a type of asexual reproduction), so their adaptability is the same, right? To understand the success of binary fission, we must also take into account that the mutation rate of the bacterial genome is very high.

For this reason, it is not always guaranteed 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 may be bad and some may be good, but the key difference is that the good ones are fixed in the population., while the negative ones disappear.

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

Summary

As you have seen, everything in nature has an explanation, except for exceptional cases. Binary fission is a reproductive strategy that is just as valid as sexual reproduction for prokaryotic organisms, since they obtain the variability genetics necessary to adapt from mutations in its 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 entire evolutionary process can be summed up in the following sentence: living things 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.

Bibliographic references:

• Eswara, P. J., & Ramamurthi, K. S. (2017). Bacterial cell division: non-models poised to take the spotlight. Annual review of microbiology, 71, 393-411.
• Binary fission, Khan Academy. Collected on March 25 in https://es.khanacademy.org/science/biology/cellular-molecular-biology/mitosis/a/bacterial-binary-fission
• Margolin, W. (2014). Binary Fission in Bacteria. eLS.
• Nystrom, 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.