BACTERIAL GENETICS: ORGANIZATION, MECHANISMS, REGULATION, TRANSFER - BIOLOGY - 2025

The bacterial genetics is the study of the bases of genetic information within the cells of the bacteria. This encompasses the organization of genetic information, how it is regulated, how it is expressed, and how it varies.

The first experiments on bacterial genetics were carried out in the 19th century, in a historical context in which it was not yet known whether bacteria had mechanisms to exchange genetic information, it was not even known whether they possessed a chromosome.

Bacterial DNA (Source: Average_prokaryote_cell-_en.svg: Mariana Ruiz Villarreal, LadyofHatsDifference_DNA_RNA-EN.svg: * Difference_DNA_RNA-DE.svg: Sponk (talk) translation: Sponk (talk) derivative work: Radio89 via Wikimedia Commons)

The only real certainty was that bacteria could establish stable lines with different phenotypes, at least for the assimilation of different nutritional compounds, and that occasionally new forms emerged, apparently due to genetic mutations.

With the great uncertainty that existed about bacteria at the time, it was imperative to answer certain questions about "bacterial genetics" experimentally, especially to understand whether bacteria met the basic principles of heredity.

Finally, in 1946, Joshua Lederberg and Edward Tatum solved these basic questions using two strains of the Escherichia coli bacteria, strain A and strain B, each with different nutritional requirements.

Type A and B cells were unable to grow in a minimal medium, since both had mutations that prevented them from assimilating the nutrients from said medium.

However, when A and B were mixed for a few hours and subsequently seeded onto the minimal medium plate, a few colonies appeared on the minimal medium plates, i.e. they grew.

These colonies originated from individual cells that had exchanged genetic material and, after the exchange, were able to express the genetic information in the phenotype and thus assimilate the nutrients from the minimal medium.

Organization of genetic information

All the genetic information necessary for the life of a bacterium is found within the "bacterial chromosome", a single molecule of double-stranded deoxyribonucleic acid (DNA).

This DNA molecule is arranged in a circular structure, closed by covalent bonds, and forms, together with some proteins, the bacterial chromosome.

Bacteria, in addition to the bacterial chromosome, can possess extrachromosomal DNA fragments of smaller size, but also structured in a closed circular way. These DNA molecules are collectively called "plasmids" or "plasmid DNA."

Plasmid DNA molecules are used by bacteria to exchange very particular genetic information between them.

Generally, when one of the bacterial cells develops resistance against an antibiotic, it can transmit that resistance to the other bacterial cells through plasmids.

The size of the plasmid DNA molecule in bacteria can vary from 3 to 10 kilo bases and in many species of bacteria hundreds of copies of a single type of plasmid can be found.

The composition and structure of the DNA of bacteria is the same as that found in all living things and in viruses. Its structure consists of a sugar skeleton, nitrogenous bases, and phosphate groups.

The complete map of the bacterial chromosome of Escherichia coli was obtained in 1963. It detailed the exact position of approximately 100 genes, but today it is known that the E. coli chromosome contains more than 1000 genes and is 4.2 in size. million base pairs.

Mechanisms of gene expression

The mechanism of gene expression of bacteria is similar in some respects to the process of gene expression that occurs in other living beings and also depends on the processes of transcription and translation.

The information from genes is transcribed to an RNA molecule and subsequently to the sequence of amino acids that make up proteins. This process is what carries out the expression of the information contained in the genotype and the structure in the phenotype.

Transcription

In transcription, the RNA polymerase enzyme creates a complementary product to a DNA segment that it uses as a template, but this product is ribonucleic acid (RNA).

This molecule carries the information for the synthesis of the protein encoded by the DNA segment, it is a single band and is called messenger RNA. The RNA polymerase of bacteria is different in bacteria and in eukaryotic organisms.

RNA polymerase identifies a specific site on DNA (promoter) where it binds to initiate transcription. A single messenger RNA molecule can contain the information for more than one gene.

Unlike eukaryotic organisms, the genes of bacteria do not have "introns" in their sequence, since bacteria do not have a nucleus that separates the chromosome from the other elements of the cytoplasm.

Translation

As all the elements are "loose" in the bacterial cell cytoplasm, the newly synthesized messenger RNA molecules can come into contact with the ribosomes and initiate protein synthesis immediately.

This allows bacteria to have an advantage in responding and adapting to extreme changes in the environment.

Ribosomal RNA, transfer RNA, and various ribosomal proteins participate in translation. The ribosomes of prokaryotic cells vary in structure and composition relative to the ribosomes of eukaryotic cells.

These elements “read” in the form of nucleotide triplets (codons) the instructions embodied in the genetic code of the messenger RNA molecules and, at the same time, they assemble each of the amino acids to form the polypeptide.

The “universality” of the genetic code allows scientists to use the translation of bacteria as an important tool for the synthesis of peptides and proteins with technological interests.

Regulation of gene expression

The mechanism that controls gene expression in bacteria is extremely accurate; it allows them to precisely regulate the amount and timing of gene product synthesis, so that they occur only when necessary.

A region of the bacterial genome that groups several genes together is called an "operon." This region activates or deactivates its transcription depending on the conditions in which the bacterium is.

All genes that are part of the same operon are coordinately transcribed into messenger RNA that contains many genes (called "polycistronic" RNA). These RNAs are translated on ribosomes sequentially, one after the other.

Operons can be positively or negatively regulated. Genes only stop expressing themselves when inhibitory proteins called repressors bind to a specific sequence in their structure.

The specific sequence of the gene is called "promoter", when the repressor protein is bound to the promoter, RNA polymerase cannot initiate transcription of the genetic sequence in question.

On the other hand, when operons are up-regulated, transcription of that genetic region will not begin until an activator protein is present that binds to the specific DNA sequence.

Scientists use this "inducibility" of operons to increase or decrease the gene expression of certain regions of interest in bacteria. By introducing some substrates, the expression of the enzymes necessary for metabolism can be increased.

Gene transfer

Bacteria, unlike eukaryotic cells, do not transfer their genes through sexual reproduction; instead, they can do so by three different processes: transformation, transduction and conjugation.

Horizontal gene transfer in bacteria (Source: 2013MMG320B via Wikimedia Commons)

Transformation

In transformation , some bacterial cells in the population become "competent". Once "competent" they are able to receive exogenous DNA from other bacteria found in the extracellular environment.

Once the DNA has been incorporated into the cell interior, the bacteria carry out a process of combining the genes contained in their chromosome with the foreign DNA that has just been incorporated within it. This process is known as genetic recombination.

Transduction

In transduction, bacteria incorporate DNA from other bacteria into their DNA molecule through viruses that infect bacteria (bacteriophages). This can be given in a specialized or generalized way.

In specialized transduction, it occurs when a phage that previously infected another bacterium acquires its genes during the infectious cycle.

Later, by infecting a new bacterium and incorporating its genes into the chromosome of the new infected bacterium, it also incorporates genes from the bacterium it previously infected.

During generalized transduction, defective phage particles that have their empty capsids incorporate part of the bacterial chromosome during viral replication, then, once they infect another bacterium, they can introduce the genes taken from the previous bacterium.

Conjugation

In conjugation, bacteria exchange genetic material in a unidirectional way, through physical contact. One of the bacteria acts as a donor and the other as a recipient. In this process, the donor bacteria generally give a plasmid DNA molecule to the recipient bacteria.

Conjugation in bacteria is not typical of all species; the conjugation capacity is granted through genes that are transmitted through a plasmid DNA molecule.

References

1. Braun, W. (1953). Bacterial Genetics. Bacterial Genetics.
2. Brock, TD (1990). The emergence of bacterial genetics (No. 579: 575 BRO). Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press.
3. Fry, JC, & Day, MJ (Eds.). (1990). Bacterial genetics in natural environments (pp. 55-80). London: Chapman and Hall.
4. Griffiths, AJ, Wessler, SR, Lewontin, RC, Gelbart, WM, Suzuki, DT, & Miller, JH (2005). An introduction to genetic analysis. Macmillan.
5. Luria, SE (1947). Recent advances in bacterial genetics. Bacteriological reviews, 11 (1), 1.