Genetic Recombination in Prokaryotes- Definition, Types, Diagrams

Genetic Recombination in Prokaryotes: Do you remember Dolly, the sheep! Dolly is not any fairy tale creature, but she was the first-ever cloned animal. Clones are produced asexually and are genetically identical to their parents. Producing a clone of a multicellular organism is considered an achievement, but bacteria are continually cloning themselves. Bacteria mostly divide asexually by binary fission and produce their clones.

Sexual reproduction is mostly absent in prokaryotes which reduces the chance of variation. However, bacteria can undergo genetic recombination in three ways: conjugation, transduction and transformation. Genetic recombination in bacteria can be in vivo or in vitro. Genetic recombination has so many applications in modern biotechnology. Read further to learn much more interesting things about genetic recombination in prokaryotes.

Genetic Recombination in Prokaryotes

Genetic recombination is the exchange of segments between two DNA molecules to form a new gene or chromosome. There are mainly two types of gene transfer.

1. Vertical gene transfer: Vertical gene transfer refers to the transfer of genetic material from parents to offspring. In bacteria, vertical gene transfer produces clones of the bacteria as bacteria reproduce by binary fission. 
2. Horizontal gene transfer: Horizontal gene transfer refers to the transfer of genetic material between two bacteria of the same strain or two bacteria of different strains or even between different species. Horizontal gene transfer leads to recombination. 

Genetic recombination forms a new gene which helps in the formation of a new gene product. The new gene product obtained due to recombination is called recombinant product. Gene recombination is often confused with mutation, but they are very different.  Gene recombination gives the desired result, whereas mutation is mostly undesired and does not produce a specific result. 

Types of Genetic Recombination in Bacteria

Gene recombination in bacteria is achieved in 3 main ways:

1. Conjugation

Conjugation is often referred to as sexual reproduction in bacteria. The process was first postulated by Joshua Lederberg and Edward Tatum. It involves direct physical contact between two bacteria via a pilus. Conjugation is the horizontal exchange of genetic material between two bacteria. Donor cells are considered male and represented as F⁺. The donor cell contains a conjugation plasmid. F plasmid, where F stands for fertility factor, is an example of a Prokaryotic conjugation plasmid. Recipient cells are considered female and represented by F^–.

The donor and recipient form temporary attachment by pilus. Although pilus connects two bacteria, a separate conjugation tube is formed by membrane fusion for genetic transfer. Conjugation tube formation is triggered by the action of an enzyme traD. The F-plasmid of the donor cell opens at the origin of replication. A nick is produced at the origin of replication. Once conjugation is initiated, the F plasmid undergoes replication and transfers the newly synthesised single strand of DNA to the recipient cell at the 5’ end and enters the recipient cell. The enzyme DNA polymerase present in the recipient then synthesises a complementary strand to produce the two-stranded DNA structure.

Fig: Conjugation in Bacteria

In some bacteria, the donor can also transfer chromosomal DNA along with the F plasmid, and the recipient combines the donor DNA with its own genome. These bacteria are known as Hfr (high frequency of recombination) strain, and the transferred genetic material is called the Hfr factor that acts as an episome. During Hfr conjugation,  the  F  factor is seldom transmitted. 
In some instances, F factors take some part of chromosomal DNA during the trader of genetic material. The F-factor with a segment of chromosomal DNA is called F’-factor. When the recipient bacteria receives the F’ – factor, the process is called sexduction.

2. Transduction

Transduction is the special type of horizontal genetic transfer which involves bacteriophages to transfer DNA from one bacterium to another. This was discovered by Joshua Leaderberg and Nortor Zinder. Two bacteria do not come into direct physical contact for the genetic transfer. Transduction is of two types.

1. Specialised transduction: When a phage gets attached to a bacterial cell wall, and the genetic material of the bacteriophage is transferred into the cytoplasm of the host bacterium, It does not cause the lysis of the host and enters the lysogenic cycle—the phage nucleic acid codes for the synthesis of the repressor proteins which prevents viral replication. With time, the phage genome enters the lytic cycle and synthesises new phages, which contain components from bacterial genome fragments. Now when this new phage infects a new bacteria, they cause genetic recombination.

2. Generalised transduction: It is a more common mode of transduction. When a phage gets attached to the bacteria,  it hijacks the machinery of the host for synthesising DNA RNA and proteins. Phage immediately starts synthesising new viral particles while fragmenting the DNA of the host bacteria. Some of the bacterial chromosome fragments attach to the viral genome, and when they affect another bacteria, recombination occurs.

Fig: Transduction of Bacteria

Prokaryotes- During the lytic cycle, viral DNA excises itself from the bacterial chromosome to produce a new virus. The process of separation is imprecise and bacterial genes get attached to the new viral DNA. The virus replicates to form many copies of the virus genome along with any host genes. The virus ruptures the cell releasing new virus particles that infect new bacterium. The continuous transfer of phage from one bacterium to another causes a combination of genes from one host to another.

3. Transformation

Transformation is defined as a kind of genetic recombination in which the DNA of the donor cell is transferred to the host bacteria in vitro. It was first described by Frederick Griffith. It is a horizontal genetic transfer in which bacteria take up plasmids from their environment and incorporate them into their genome. The bacterium is brought into the state of competence by methods like electroporation, or heat shock or chemical treatment. This allows the bacteria to uptake plasmids from the environment.

Transformation can be natural. During unfavourable environmental conditions, some bacteria release DNA into the environment, which can be picked by competent bacteria causing natural transformation.
Transformation experiments are usually performed among bacteria of the same species. Usually, \(1\) to \(10\,{\rm{Kb}}\) of DNA can be transferred by transformation experiment. Transformation is performed to introduce foreign DNA into bacteria by placing it into the environment. In vitro transformation is highly specialized and requires cloning vectors such as plasmids with specific DNA sequences or genes. 

Once the bacteria uptakes the foreign DNA, the bacterial colony is screened based on antibiotic-resistant property or the colour of the colony. The inserted DNA does not necessarily need to be integrated into bacterial chromosomes; it can self-replicate autonomously.

Fig: Transformation in Bacteria

Summary

Genetic recombination is the exchange of segments between two DNA molecules resistant to form a new gene or chromosome. Genetic transfer occurs horizontally. Genetic recombination forms a new gene, which produces a new gene product called recombinant product. Gene recombination in bacteria is achieved in three main ways: conjugation, transduction and transformation. Conjugation is referred to as sexual reproduction in bacteria. The donor is considered male or F⁺, and the recipient is considered female or F^–. The donor and the recipient form temporary attachments for genetic transfer by pilus called the conjugation tube.

The F-plasmid of the donor cell undergoes replication and transfers the newly synthesised single strand of DNA to the recipient cell. In Hfr strains of bacteria, the donor can also transfer chromosomal DNA, and the recipient combines the donor DNA with its own genome. This process is called sexduction. Transduction is the special type of horizontal genetic transfer which involves bacteriophages to transfer DNA from one bacterium to another.  The virus ruptures the cell releasing new virus particles that infect a new bacterium. Transformation is defined as a kind of genetic recombination in which the DNA of the donor cell is transferred to the host bacteria in vitro. Transformation experiments are usually performed among bacteria of the same species. Transformation is performed to introduce foreign DNA into bacteria by placing it into the environment.

Frequently Asked Question (FAQs) on Genetic Recombination of Bacteria

Q.1. What are the three mechanisms of genetic recombination in prokaryotes?
Ans: Conjugation, transduction and transformation are the three mechanisms of genetic recombination in Prokaryotes.

Q.2. What is the importance of genetic recombination in bacteria?
Ans: Genetic recombination in bacteria provides bacteria adaptability to the environment. It also helps in obtaining desired products like medicine and vaccines, etc.

Q.3. What is genetic recombination?
Ans: Genetic recombination is the exchange of segments between two DNA molecules resistant to form a new gene or chromosome.

Q.4. Do prokaryotes undergo recombination?
Ans: Yes! Prokaryotes undergo recombination.

Q.5. Name the scientist who discovered the phenomenon of transduction.
Ans: Joshua Leaderberg and Nortor Zinder. 

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