Transduction in bacterial cells is a type of genetic recombination in which a piece of chromosomal DNA is transported from one bacterial cell (referred to as the donor) to another bacterial cell (the recipient) by a bacteriophage (the bacterial virus that is also known as a phage).
Essentially, genetic recombination in bacteria refers to a process through which a segment of DNA is transferred from one bacterial cell to another thus allowing for genetic reshuffling in the recipient cell. This can be beneficial given that the genetic material received (by the recipient) can produce favorable traits for survival. During transduction, a bacteriophage acts as the vehicle through which this process is successfully completed.
* Bacterial transduction was first described in 1952 by Norton Zinder and Joshua Lederberg.
* The word "transduction" means to carry across and only a small fragment of DNA (between 50 and 100kb in length) is transferred to the recipient.
Bacterial genetics is a subdiscipline of genetics that is concerned with the study of genetic material of these organisms, information stored in the genes, expression of this information as well as the transfer of this information from one cell to another etc.
Before looking at some of the main steps involved in transduction in bacterial cells, it's important to understand some areas of bacterial genetics.
Bacterial Genome and Genome Organization
The genetic information in bacteria exists in the form of a circular molecule of DNA that is capable of self-replication. Apart from this chromosomal DNA (which may be described as the essential replicon), bacterial cells may also possess a number of extrachromosomal genetic elements including plasmids etc which are associated with such functions as producing virulence factors and conferring antimicrobial traits to the organisms etc.
As is the case with the double-stranded DNA of many other organisms, bacterial DNA is helical with the two strands being antiparallel to each other. Each of the strands consists of repeating units of deoxyribose and phosphate groups as the backbone as well as repeating nucleosides which consist of nucleobases and ribose.
* The DNA helix (consisting of strands) is stabilized by hydrogen bonds that are located between complementary base pairs.
* Compared to the genome eukaryotic cells. The bacterial genome is smaller in size and tends to be less variant (ranging between 139kbp to 13,000kbp depending on the type of bacteria).
* In bacteria (and prokaryotes in general) the DNA is not contained in a membrane. Here, the chromosome, as well as RNA molecules and a number of proteins are localized in the cytoplasm and make up the nucleoid.
The majority of bacteria are characterized by a covalently closed, circular chromosome which is different compared to the linear chromosomes eukaryotes. However, some bacterial cells have been shown to possess multiple circular chromosomes (e.g. Brucella and Borrelia etc) or linear chromosomes as well as linear plasmids.
Bacteriophages (also known as bacterial viruses or phages) are not necessarily extracellular DNA of bacteria. However, they contain nucleic acid material and replicate within the bacterial cells as obligate parasites.
Generally, bacteriophages are metabolically inert and thus lack essential organelles required for such activities. For this reason, they are often referred to as particles.
In addition to the nucleic acid (DNA, RNA, or both depending on the bacteriophage), they also contain proteins that provide a protective covering known as the capsid. Bacteriophages are divided into two main groups which include virulent and temperate phages.
In a susceptible bacteria, an infection by either type of bacteriophage results in the death of the host (bacteria). However, in some cases, a type of temperate growth (by temperate phages) known as Lysogeny results in the genome of the virus replicating as a prophage.
In this case, the bacterium does not succumb to the infection. However, in the event that the genome is activated to produce phages, they can become lytic and destroy the bacteria.
Exchange of Genetic Material (Horizontal Gene Transfer)
For bacteria, exchanging genetic material/information is a form of sexual reproduction given that the process ultimately results in the production of offspring with a combination of genetic material from different bacterial cells. This exchange has been shown to accelerate the evolution of genomes as compared to mutation.
New genetic information from the donor introduces new traits to the recipient that may prove beneficial depending on the type of environment in which the organism exists. Here, it's worth noting that there are several methods through which bacteria can exchange genetic information.
1/ Transformation - where the recipient bacterial cell takes up free DNA fragments in their surrounding
2/ Conjugation - where a temporary union between two bacterial cells results in the exchange of genetic material
3/ Transduction - focusing as a means of transfer
Steps of Transduction
As mentioned, transduction in bacterial cells refers to the process through which a viral particle transfers or transports genetic material from one bacterial cell to another bacterial cell. Following this transfer, the recipient is commonly known as the transductant.
Although bacteriophages are key players in transduction, it's worth noting that not all bacteriophages are capable of this. To participate in a successful transduction process, a bacteriophage should be able to degrade the appropriate level of chromosomal DNA so as to produce the right size of fragments for packaging before it can be transferred.
Although transduction generally involves the transfer of genetic material from one bacterial cell to another via viral particles, there are two main types of transduction that include:
One of the most important steps of transduction in bacterial cells involves the packaging of DNA material into the head region of the phage/bacteriophage so that it can be carried and transferred to a recipient bacterial cell.
For the most part, the packaging process is highly specific for phage DNA which means that DNA belonging to the phage is mostly packaged into the head of the particle. In some cases, an error may occur resulting in the packaging of small fragments of the bacterial DNA into the head region of the phase (rather than phage DNA). In this case, it's the bacterial DNA fragments that are transferred to the recipient.
This type of generalized transduction follows the following steps:
Physical contact with a bacterial cell - The first step involves physical contact and attachment of the viral particle/bacteriophage to the target bacterial cell. This is made possible by the phage receptors (these are bacterial surface molecules including proteins and lipopolysaccharides etc that are essential for various other functions).
Following this contact, the bacteriophage injects its genetic material (DNA content) into the bacterial cell given that the replication of viral DNA molecules can only occur within infected cells.
Replication - In the cytoplasm, the DNA molecule is first transported to the nucleoid where it can take advantage of the cell's existing transcriptional machinery. Here, phase DNA produce phage enzymes that break down the bacterial DNA into small fragments.
Taking advantage of the existing replication machinery of the cell, phage DNA is also replicated while phage proteins are produced through the translation of phage mRNA information. Following successful replication of phage DNA and production of phage proteins, the process enters the next step.
Assembly and Packaging - Following DNA replication and protein production, the assembly process results in the formation of new viral particles where the replication DNA molecules are packed into the head of the phage in the capsid. During this process, some of the phage heads may end up surrounding some of the bacterial DNA fragments rather than viral DNA molecules.
With increased virion production within the bacterial cell, the cell is ultimately destroyed (during the lytic cycle) as the virions are released from the cell.
Infection of the recipient bacterial cell - The next step of generalized transduction involves infection of the recipient bacterial cell by the phage carrying DNA fragments from the donor bacterial cell (that was destroyed in the step above). This infection transfers bacterial DNA to the new/recipient cell where it's integrated/introduced into the bacterial chromosome where it introduces new genes.
As the new bacterial cell multiplies, the daughter cells will retain the foreign genetic material and pass them over to new generations.
Generalized transduction may be presented as follows:
a. Bacteriophage injects viral DNA into the bacterial cell
b. Production of phage enzyme breaks down bacterial DNA into small fragments
c. Viral DNA is replicated
d. New virions start to assemble and some bacterial DNA fragments (in red) are surrounded by the virion capsid
e. New virions leave the infected cell
f. Viral particle carrying bacterial DNA (from the donor bacterial cell) infects new bacterial cell
g. DNA fragment from the donor bacterial cell is introduced/integrated into the DNA of the recipient bacterial cell. When this cell divides, daughter cells will also retain the inserted DNA fragment
In specialized transduction in bacterial cells, the phage also comes in contact with the bacterial host and injects its DNA into the bacteria. However, unlike generalized transduction, the phage enters the lysogeny cycle rather than producing enzymes that can destroy the bacterial DNA.
The viral genome is integrated into the chromosome of the host bacteria and is duplicated as the bacterial DNA is replicated. Under certain conditions (e.g. changing environmental conditions) the phage particles are produced which means that the replicated viral DNA leaves the host's chromosome.
The viral DNA may retain a piece of the host's DNA while the host's chromosome retains pieces of viral DNA. While the phages that develop through this process are defective (they do not have the entire viral genome), they still destroy the infected cell and can infect new bacterial cells.
Following infection of a new bacterial cell, the viral DNA (which consists of pieces of the former bacterial DNA) is integrated into the chromosomal DNA of the new bacterial cell. As a result, the recipient bacterial cell will not only receive viral genome but also genome from the other host bacteria.
Like transduction, transformation is a type of horizontal gene transfer (lateral gene transfer) through which the genetic material of one bacterial cell is integrated into the chromosomal DNA of another bacterial cell. The mechanism through which this process takes place is different between the two.
There are few similarities between transduction and transformation. Both processes involve gene transfer from one bacterial cell to another. This means that genetic material has to be from one bacterial cell and transferred to another bacterial cell.
Both processes ultimately end up with the new genetic material, from the donor, being integrated into the chromosomal DNA of the recipient bacterial cell.
Differences between Transduction and Transformation
One of the main differences between transduction and transformation in bacterial cells is with regards to the mechanism through which genetic material is transferred from one cell to another. As already mentioned, genetic material from the donor bacterial cell to the recipient bacterial cell in transduction is dependent on bacteriophages.
Here, bacteriophages act as agents that carry genetic material from the donor and transport it to the recipient cell. In transformation, a transport agent is not required for this process. Rather, the recipient bacterial cell takes up the genetic material of other bacterial cells from their surroundings.
In a case where given bacterial cells undergo lysis so that their genetic material is released into the surrounding environment, DNA fragments (may be fragments from chromosomal DNA of the bacteria or plasmids) may be taken up by other bacterial cells in their surroundings. This has been observed in different types of bacteria including Streptococcus, Bacillus, and Staphylococcus among others.
Given that there is no contact between two bacterial cells or between a bacterial cell and a bacteriophage, type IV pilus system plays an important role in the uptake of DNA from the environment.
Following the uptake, the genetic material is then integrated into the chromosomal DNA of the recipient bacterial cell which can have a number of beneficial outcomes (e.g. conferring resistance traits against antibiotics or allowing the bacterial cell to produce toxins, etc).
The other difference between transduction and transformation in bacterial cells is the fact that the uptake of genetic material during transformation is not characterized by a lytic or lysogenic cycle.
In transduction, viral activities (e.g. production of enzymes and increased replication) have been associated with the destruction of the donor bacterial cell and recipient bacterial cells in some cases. However, given that bacteriophages are not involved in transformation, these cycles do not take place.
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