Transduction is a biological phenomenon by which genetic material (DNA) is transferred from one bacterium to another by a virus. Viruses that transfer bacterial genetic material are called bacteriophages.
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How was viral transduction discovered?
The process of viral transduction was discovered by Joshua Lederberg and Norton Zinder in 1951. While studying the recombination process using two different strains of Salmonella typhimurium, the scientists inhibited the conjugation-based cell contact between two strains using a filter and found that the recombination was carried out by a bacteriophage called P22.
The scientists termed the virus-mediated gene transfer process as transduction. Both virulent and temperate phages can conduct the transduction process.
What is the mechanism of viral transduction?
The viral transduction process can be categorized into two forms: generalized and specialized. During generalized transduction, any part of the bacterial genome is transferred to the new host, whereas only specific parts of the bacterial genome are transferred during specialized transduction.
Bacteriophages utilize bacteria as hosts and start replicating inside the host (bacterial) cells. Mechanistically, when a bacteriophage lyses a bacterium, the bacterial genome is fragmented into small parts. During viral replication, a small part of the bacterial genome is sometimes mistakenly incorporated into the viral genome. This initiates the process of viral transduction.
When the transducing bacteriophage attacks and subsequently injects its genomic content into a new host bacterium, a partially diploid bacterium in generated wherein the transduced part of the bacterial genome is incorporated into the genome of the new host through recombination. This entire process is called generalized transduction.
During specialized transduction, a bacteriophage selectively picks up a specific part of the host genome that is located adjacent to the designated prophage incorporation site. The genome of a bacteriophage that is integrated into the host genome is called a prophage. One classical example of a bacteriophage involved in specialized transduction is lambda (ʎ), which, as a prophage, is incorporated between the gal and bioregions of the host genome.
Upon induction of the lytic cycle, in addition to the production of a normal circular ʎ genome, abnormal transducing particles can rarely be formed that contains gal genes on one side and bio genes on the other side. This modified ʎ phage is called ʎ-defective gal (ʎdgal) or a ʎ-defective bio (ʎdbio). This defective phage containing adjacent genes can transfer the genetic material into a newly infected second host cell.
What are the characteristics of a bacteriophage involved in transduction?
There are two types of bacteriophages: virulent phages and temperate phages. For reproduction, virulent phages always rely on the lytic cycle wherein the phage utilizes host cell machinery for DNA replication and new protein formation. Afterward, the host cell membrane is disrupted by the phage proteins, and newly formed phages are released into the environment to continue the cycle of infection.
In contrast, temperate phages mostly utilize the lysogenic cycle for reproduction. In certain conditions, however, temperate phages can also initiate a lytic cycle. During a lysogenic cycle, a bacteriophage remains within the host cell as a prophage, which in turn protects the host cell from secondary infections and ensures its survival as a lysogenic bacterium.
A prophage is duplicated within the host cell and transferred to the next progeny through cell division.
Under certain circumstances, a prophage can enter the lytic cycle. When activated by certain agents, such as ultraviolet rays or chemicals, a prophage induces host cell lysis and triggers the release of infective phages into the environment.
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What are the applications of viral transduction?
There are several applications of the viral transduction process in basic biology as well as clinical fields.
The most important application of viral transduction can be seen in the experiments conducted by Alfred Hershey and Martha Chase in 1951 – 1952. They labeled bacteriophage proteins and DNA with radioactive sulfur and phosphorus, respectively, and conducted a series of experiments (commonly known as Hershey-Chase experiment) to prove that DNA is the only genetic material that enters a host cell and replicates. These experiments helped scientists form the basis of inheritance.
Bacteriophages involved in generalized transduction have been utilized extensively for the mapping of the bacterial genome. Given the fact that bacteriophages can pick up and transduce two closely located genes in a single DNA fragment, generalized transduction can be utilized to evaluate the association between genes on the same bacterial chromosome (linkage analysis). By measuring the frequency of cotransduction, the distance between two chromosomal genes can be calculated.
Using the basic transduction process, several techniques have been developed to transfer genes into cultured cells or in animals and study gene functions. The most commonly used viral vectors for gene transfer include retrovirus, lentivirus, adenovirus, adeno-associated virus, and Herpes simplex virus. All these vectors are capable of maintaining a stable transgene expression for a prolonged period.