Horizontal Gene Transfer

The movement of genetic information between organisms is known as gene transfer. Gene transfer can be done by 2 methods, horizontal gene transfer, and vertical gene transfer. Horizontal gene transfer can be described as the transfer of genetic information between two independent organisms. It is also known as lateral gene transfer. It can be between the same species, different species. It can also occur between the bacterial species and eukaryotic species. Vertical gene transfer is the transfer of genetic information from parent to progeny. Horizontal gene transfer in humans and other multicellular is a rare event.

Horizontal Gene Transfer in Bacteria

The transfer of genetic data from bacteria to another bacteria or other viruses is known as horizontal gene transfer in bacteria. There are some general features of gene transfer in bacteria, they are as follows-

1. There is no direct cell-to-cell contact.

2. Naked DNA is transferred.

3. There is no polarity in transfer, that is gene transfer is bidirectional (any one of the bacteria can act as a donor or recipient)

4. DNA which is transferred is sensitive to DNase activity. (deoxyribonuclease enzyme can degrade the transferred DNA)

For the successful transfer of genetic information, the bacteria must be competent. Competence can be defined as the ability of the recipient bacterium to take up naked DNA from the surrounding environment. The DNA that enters into the host’s cell is known as exogenate.

Generalized Mechanism of Lateral Gene Transfer

Endogenote from species X is replicated and excised. Now, this exogenote enters the cell of bacteria of the same species X. The recipient cell in this stage, when the exogenote is not incorporated into the host genome is known as, merozygote, partial diploid, or merodiploid.

There are two possible fates of exogenote, they can either incorporate into the host genome or can be degraded by the host enzyme, DNase. Those cells which incorporate the exogenote are termed recombinant cells. The probability of exogenous incorporation increases if the exogenate shares homology or is derived from the same species.

Types of Horizontal Gene Transfer

Gene transfer in bacteria was studied by Griffith in 1928, there are the following types of gene transfer mechanism, conjugation, transformation, and transduction.

Transformation

Transformation is studied in Streptococcus pneumoniae, in this type of gene transfer in bacteria, donors can be of the same or different species, which means it is not species-specific. Sequence non-specific linear DNA uptake takes place. Only single-stranded DNA  enters the recipient cell because endonuclease A cleaves one strand of the DNA. After the entry of the exogenote recipient, in merodiploid form. The exogenous can now incorporate into the host genome for successful transformation. This leads to the formation of recombinant cells. If the exogenate does not incorporate into the host genome it is degraded by host enzymes such as DNase. In later cases, transformation is concluded to be unsuccessful.

Conjugation

Conjugation was proposed by Lederberg and Tatum in 1946. It requires a direct cell-to-cell contact, non-naked DNA is transferred. Unlike transformation, conjugation shows polarity which means only one of the cells can act as a donor. The cell that has an F plasmid (fertility plasmid) can only act as a donor. They are represented as F+. The recipient cells lack F plasmid, and are denoted as F-. In this type of conjugation, only plasmid DNA is transferred. Mechanism of conjugation is as follows

1. Donor cell which has fertility plasmid attaches to the recipient cell through pilus.

2. This attachment brings the recipient close to the donor

3. Exogenic is excised out

4. Single-stranded DNA moves through the pilus to the recipient cell

5. A complementary strand of the exogenate is synthesized in the recipient, the recipient now has a complete fertility plasmid.

6. The donor also synthesizes the complementary strand and returns to its original state. 

These conjugation results can be written as F+ × F- → F+ × F+ which implies that after conjugation both the cell has fertility plasmid.

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Transduction

It is the process of horizontal transfer of genetic information mediated by viruses. Bacteriophages, viruses that attack bacteria, carry genes from one bacteria and transfer them to the other bacteria. Bacteriophage uses bacteria as their host and during their infection cycle, they transfer the genetic material of bacteria to another host. Transduction does not need direct cell-to-cell contact. In it part of chromosomal DNA is transferred, it does not have the polarity of transfer. This process is DNase insensitive. There are two types of transduction- generalized and specialized transduction.

Generalized Transduction

In this type of horizontal gene transfer in bacteria, non-specific bacterial chromosomal fragments get transferred into the recipient. It is reported in P1 phage, which uses E.coli as host, P22 phage which uses salmonella as its host. The steps of generalized transduction are as follows-

1. Bacteriophage attaches to the bacterium surface

2. It then introduces viral genetic material into the host genome

3. Replication of viral genetic material using the host's enzyme takes place simultaneously and the bacterial chromosome gets fragmented.

4. Packaging of viral genetic material in capsids but at random, some of the bacterial chromosomal fragments get packaged into the capsid.

5. Bacterial cells now undergo lysis, releasing bacteriophages. These bacteriophages will infect new host bacterium thus transferring bacterial genetic material along with its genetic material to the new host.

This subsequent infection creates a partial diploid condition which can either integrate or get degraded. If the chromosomal fragments do not integrate with the bacterial genome it undergoes degradation and such transduction are known as abortive transduction.

Specialized Transduction

It is also known as restricted transduction, in this type of horizontal transfer only a specific portion of the bacterial genome is transferred by the phage particle. It occurs only in pages that follow the lysogenic cycle and then enter the lytic state. It is observed in lambda phage which uses E.coli as the host. At first lambda phage attaches to the bacterium then it injects its genetic material into the bacterium. The viral genome then integrates into the bacterial genome and undergoes replication, this is known as prophage. Bacteria remain in the lysogenic state for some time, after which it enters the lytic phase, infecting another host.

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