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Bacteria are prokaryotic organisms that reproduce asexually. The most frequent form of reproduction in bacteria is through the process called binary fission. Binary fission is the division of a single cell, resulting in the formation of two cells that are genetically identical . In order to understand the binary fission process, it is necessary to know the cell structure of bacteria, and that is the objective of the first section of this article.
The cell structure of bacteria
Bacteria can have various forms. The most common are spherical, rod-shaped, and spiral-shaped. In general, bacteria are composed of a cell wall, cell membrane, cytoplasm, flagella, nucleus, plasmids, and ribosomes. Let’s see what it is and what function each of these components has.
- cell wall . It is the outer covering of the cell, which protects it and gives it its outer shape.
- cytoplasm . It is a gelatinous substance composed mainly of water and containing enzymes, salts, various cell components, and various organic molecules.
- Cell membrane or plasma membrane . It is a membrane that surrounds the cytoplasm and regulates the flow of substances in and out of the cell.
- Flagella . They are a series of long, whip-shaped protuberances that cause the cell to move.
- Ribosomes . They are the cellular structures that are found in the cytoplasm and are responsible for the synthesis of proteins from the information that DNA has, and that reaches the ribosomes through messenger RNA (ribonucleic acid).
- plasmids _ They are gene-carrying structures, formed by DNA (deoxyribonucleic acid) that acquires a circular shape and does not participate in cell reproduction.
- Cell nucleoid . It is the area of the cytoplasm that contains the single DNA molecule of the bacterium.
binary fission
Most bacteria, including Salmonella and Escherichia coli (both very important bacteria for humans, as they cause serious diseases), reproduce by binary fission. In the process that takes place in this type of asexual reproduction, the single DNA molecule of the bacterium is replicated, and both copies adhere to different points on the cell membrane. The cell then begins to grow and elongate, thus increasing the distance between the two DNA molecules. When the cell nearly doubles its original size, the cell membrane begins to collapse toward the center of the cell. Eventually, a cell wall forms, separating the two DNA molecules and dividing the original cell into two identical daughter cells.
Binary fission has several advantages as a form of cell reproduction. A single bacterium is capable of generating a large number of new cells, with a high speed of reproduction. Under optimal conditions, some bacteria can double their population numbers in a matter of hours, or even minutes. Another advantage is that no time is “wasted” looking for a mate, since reproduction is asexual. In addition, the cells resulting from binary fission are identical to the original cell, which implies that they are suitable for living in that environment.
bacterial recombination
Binary fission is an efficient form of reproduction for bacteria; however, this form of reproduction could lead to certain problems for a colony of bacteria. Since the cells produced through this type of reproduction are identical, they are all susceptible to the same types of threats, such as environmental changes or antibiotics. These situations could destroy an entire colony of bacteria. To avoid these dangers, bacteria can introduce genetic variations in recombination processes . Recombination involves the transfer of genes between cells. The recombination processes of bacteria can occur in three ways: by conjugation, by transformation, and by transduction.
The conjugation process occurs in some bacteria that are capable of transferring their genes to other bacteria with which they come into contact. During conjugation, one bacterium connects to another through a tube-like structure made of protein called a pilus . Genes are transferred from one bacterium to another through this tube.
Some bacteria are capable of taking up DNA from their environment, and this process is called transformation . These DNA remnants commonly come from dead bacterial cells. During transformation, the bacterium takes DNA from its environment and transfers it across the cell membrane. This “recycled” DNA is then incorporated into the cell’s DNA.
Transduction is a type of recombination that involves the exchange of bacterial DNA via bacteriophages . Bacteriophages are viruses that infect bacteria. There are two types of transduction: generalized transduction and specialized transduction.
Once a bacteriophage attaches to a bacterium, it inserts its genome into the bacterium. The viral genome, enzymes, and viral components are replicated and assembled within the host bacterium. Once formed, the new bacteriophages kill or break open the cell, thus releasing the replicated viruses. However, during the process in which the virus attaches to the bacterium, some of the host bacterium’s DNA may become encased in the viral capsid instead of the viral genome itself. When this bacteriophage infects another bacterium, it injects the DNA fragment from the previously infected bacterium. This piece of bacterial DNA is then inserted into the DNA of the new bacterium. This type of transduction is called generalized transduction.
In specialized transduction, fragments of the host bacterium’s DNA are incorporated into the viral genomes of new bacteriophages, and the DNA fragments can be transferred into any new bacteria these bacteriophages infect.
Fountain
- Reece, Jane B., and Neil A. Campbell. Campbell Biology . Benjamin Cummings, 2011.
