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Horizontal Gene Transfer

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Biology

Did you know that genetic material can be transferred between distantly-related organisms? This process is called horizontal gene transfer. We will discuss horizontal gene transfer: its definition, types, manifestation in bacteria, importance, and detection methods.

What is the Definition of Horizontal Gene Transfer?

Horizontal gene transfer -also known as lateral gene transfer- is a general term for the non-sexual exchange of genetic material among organisms, even between distantly-related species. It is distinct from the vertical transfer, in which genetic material passes on from parent to offspring.

This distinction can be better understood through Figure 1, a phylogenetic tree that shows how genetic material moves vertically (through reproduction) and horizontally (through horizontal gene transfer).

Horizontal Gene Transfer in Phylogenetic tree

Figure 1. This diagram illustrates how genetic material moves vertically through reproduction and horizontally through horizontal gene transfer across the different domains of life. Source: Andrew Z. Colvin, CC BY-SA 4.0, via Wikimedia Commons.

Three Types of Horizontal Gene Transfer

Horizontal gene transfer occurs through transformation, conjugation, and transduction. These processes are illustrated in Figure 2 below. We will discuss each of these types in the following section.

3 Types of Horizontal Gene Transfer

Figure 2. This diagram shows how the 3 types of horizontal gene transfer occur in bacteria. Source: 2013MMG320B, CC BY-SA 3.0, via Wikimedia Commons.

Transformation

Transformation is one of the most common mechanisms of horizontal gene transfer among bacteria. Transformation happens when a recipient competent cell takes up the extracellular naked DNA from the environment or DNA from a dead donor bacterium.

Transformation involves homologous recombination, the exchange of genetic material between single-stranded or double-stranded nucleic acids that have extensive regions of similar base sequences. Donor and recipient cells are usually of the same bacterial species.

Conjugation

Another mechanism of horizontal gene transfer in bacteria is conjugation. Conjugation is the most common mechanism of horizontal gene transfer from a donor bacterial species to a different recipient species. Bacterial conjugation was first discovered in E. coli, and it relies on cell-to-cell contact. The direct contact of cells with each other is important for bacterial conjugation and distinguishes it from other mechanisms of horizontal gene transfer.

In bacterial conjugation, the donor cell would pull the recipient cell using a structure called a sex pilus. Sex pilus is a tube-like appendage that allows cell-to-cell contact, and it protects the transfer of DNA plasmid from the donor to the recipient cell. Once they are in contact with each other, DNA is pushed out of the donor cell and transported to the recipient cell.

Transduction

Transduction involves the transfer of DNA fragments through a viral vector called a bacteriophage. A bacteriophage is a virus that can infect bacteria. Bacteriophage, the transducing particle, would infect a bacterium and then inject its DNA into the bacteria. After injection, the DNA from the virus can hijack the bacterium and use it as a host to make more of the DNA. Unlike conjugation, transduction does not require physical contact between the donor cell and the receiver cell.

Transduction can happen by a lytic or lysogenic cycle. In the lytic cycle, the bacteriophage takes over the bacterial cell's replication, transcription, and translation machinery to direct it to make new viral particles. Once there are viral particles, it is released into the environment via the lysis of the host.

On the other hand, in a lysogenic cycle, the bacteriophage chromosome is inserted as a prophage into the bacterial chromosome. This prophage can remain dormant in the bacteria, but the prophage can be excised from the bacterial chromosome and initiates the lytic cycle if the bacteria is induced.

Example of Horizontal Gene Transfer in Bacteria

Bacteria are versatile organisms that can take up foreign DNA, replicate it, and pass it down to future generations. They can respond to selective pressure and adapt to their new environment via different mechanisms. They are also able to adapt because they can acquire new genetic traits. These new genetic traits are acquired either through mutation or modification of gene faction from horizontal gene transfer. We will briefly discuss these two processes in the following section.

Mutation

Mutation occurs very slowly in nature; mutation rates vary from 10-6 to 10-9 per nucleotide per bacterial generation. Other times, mutation can happen faster, such as if a bacteria population is under a lot of stress from the environment. However, most mutations in bacteria are lethal and lead to death.

Horizontal gene transfer

Through horizontal gene transfer, bacteria can adapt rapidly by acquiring DNA from another bacterium in a single transfer. Horizontal gene transfer is involved in bacterial genome evolution, DNA damage prevention, virulence, antibiotic resistance, and overall survival in different environmental stresses.

A critical aspect of horizontal gene transfer in bacteria is the exchange of antibiotic resistance genes. Exchange in antibiotic resistance genes leads to evolution and plays a role in developing multi-drug resistant bacteria that are pathologically relevant to humans. Most resistance genes rapidly evolve and spread among different human pathogens.

Detecting Horizontal Gene Transfer

The ability to detect horizontal gene transfer is important in the research and medical field. In health sciences, horizontal gene transfer has a significant role in the evolution of new pathogens, especially in promoting antibiotic resistance in bacterial species, which usually leads to multi-drug resistant pathogens. Genes that confer antibiotic resistance are horizontally transferred, for example, from a sensitive Staphylococcus aureus bacteria, resulting in a methicillin-resistant strain.

The two most commonly used methods in detecting horizontal gene transfer are the phylogeny-based approach and the composition-based approach, which utilize different bioinformatics tools.

Phylogeny-based Approach

In a phylogeny-based approach, a large set of gene copies of the gene being investigated is compared and contrasted to the phylogeny of their originating species. If there are differences between the investigated gene and the phylogeny, it is reconciled by introducing horizontal gene transfer. This approach allows us to identify ancient horizontal gene transfer events in a genome, but it only works under strict restrictions.

This approach relies on multiple sequence alignment (MSA) and the reliability and accuracy of existing phylogenetic species trees. The gene transferred should also originate from a relatively distant donor from the recipient cell, so divergence can be detected.

Composition-based Approach

In a composition-based approach, the genome is compared and contrasted based on its properties. Limitations of this approach stem from similar compositional patterns of different species. Also, the length of the gene transferred from donor to recipient may be too small to be detected.

Why is Horizontal Gene Transfer Important?

Horizontal gene transfer is essential in developing and increasing the biodiversity of organisms. It is a rapid mechanism that increases genetic variability that plays a role in adaptation, survival, and, eventually, bacterial evolution of the newly transferred genes.

Horizontal gene transfer not only occurs between similar organisms, but also among different domains from bacteria to archaea, archaea to bacteria, archaea to eukarya, and bacteria to eukarya. For example, the transfer of metabolic enzyme of Pasteurella bacteria to protozoa Trichomonas vaginalis led to the protozoa's adaptation to an animal host.

Another example of horizontal gene transfer in eukaryotes involves the transfer of fungal genes to a pea aphid (Acyrthosiphon pisum). This was the first reported carotenoid synthase gene in animals. However, most well-understood and documented horizontal gene transfers are among bacterial and archaeal evolution.

Horizontal gene transfer - Key takeaways

  • Horizontal gene transfer is the movement of genetic material between organisms. It is distinct from vertical gene transfer, in which genetic material is passed on from parent to offspring.

  • The three types of horizontal gene transfer are:

    • Transformation: bacteria take up DNA that is in the environment

    • Conjugation: bacteria in contact with each other transfers genetic material

    • Transduction: bacteriophage transfer genes from one cell to another

  • There are two commonly used methods in detecting horizontal gene transfer. These are:

    • Phylogeny-based approach: a large set of gene copies of the gene being investigated is compared and contrasted to the phylogeny of their originating species.

    • Composition-based approach: the genome is compared and contrasted based on its properties.

  • Horizontal gene transfer is important in developing and increasing the biodiversity of organisms. It is also essential for bacterial evolution and pathogen evolution through the acquisition of antibiotic resistance.

Horizontal Gene Transfer

Horizontal gene transfer is the non-sexual movement or exchange of genetic material from one organism to another. 

Yes. Conjugation is one of the three main mechanisms of horizontal gene transfer. It transfers genetic information from one organism through direct contact using the pilus. 

Horizontal gene transfer work via three main mechanisms: 

  • Transformation- bacteria take up DNA from the environment
  • Conjugation - bacteria directly transfer genetic material to another organism
  • Transduction- bacteriophage transfer genetic material to the bacteria

Horizontal gene transfer can be detected through a phylogeny-based or a composition-based approach. 

  • Phylogeny-based approach: comparing the phylogenetic tree of species to its originating species
  • Composition-based approach: investigation of the sequence composition of organisms

Horizontal gene transfer is important because it leads to adaptation, survival and evolution of organisms. Horizontal gene transfer promotes genetic variation and high biodiversity. In bacteria, horizontal gene transfer leads to strains with greater resistance to antibiotics that are clinically relevant to humans. 


Final Horizontal Gene Transfer Quiz

Question

What is horizontal gene transfer?

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Answer

Horizontal Gene Transfer is the non-sexual movement or exchange of genetic material among organisms, even between distant species.

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Question

What is conjugation?

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Answer

Conjugation is one of the three main mechanisms of horizontal gene transfer. It involves the transfer of genetic information from one organism through direct contact using the pilus.

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Question

How is horizontal gene transfer different from vertical gene transfer?

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Answer

Horizontal gene transfer involves the movement of genetic material between organisms that may be distantly related while vertical gene transfer involves passing on genetic material from parent to offspring.

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Question

What are the 3 main types of horizontal gene transfer?

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Answer

The 3 main types of horizontal gene transfer are transformation, conjugation, and transduction.  

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Question

What type of horizontal gene transfer is described when a recipient cell takes up the extracellular naked DNA from the environment or DNA from a dead donor bacterium?

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Answer

Transformation

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Question

What type of horizontal gene transfer relies on cell-to-cell contact?

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Answer

Conjugation

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Question

What is a sex pilus?

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Answer

A sex pilus is a tube-like appendage that allows cell-to-cell contact.

Show question

Question

How does bacterial conjugation work?

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Answer

In bacterial conjugation, the donor cell would pull the recipient cell using a a sex pilus. Once they are in contact with each other, DNA is pushed out of the donor cell and transported to the recipient cell.  

Show question

Question

What type of horizontal gene transfer involves the transfer of DNA fragments through a viral vector called a bacteriophage?

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Answer

Transduction

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Question

What is a bacteriophage?

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Answer

A bacteriophage is a virus that can infect bacteria.  

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Question

What is the importance of horizontal gene transfer?

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Answer

Horizontal Gene Transfer is important in developing and increasing the biodiversity of organisms. It is a rapid mechanism that increases genetic variability that plays a role in adaptation, survival, and eventual bacterial evolution of the newly transferred genes. 

Show question

Question

How are horizontal gene transfers detected?

Show answer

Answer

There are two most commonly used methods in detecting Horizontal Gene Transfer: the phylogeny-based approach and the composition-based approach.

Show question

Question

Why is detecting horizontal gene transfer important in the research and medical fields?

Show answer

Answer

The ability to detect Horizontal Gene Transfer is important in the research and medical field. In health sciences, Horizontal gene transfer has a significant role in the evolution of new pathogens especially promoting antibiotic resistance in bacterial species, which usually leads to multi-drug resistant pathogens.  

Show question

Question

What is the phylogeny approach to detecting horizontal gene transfers?

Show answer

Answer

In a phylogeny-based approach, a large set of gene copies of the gene being investigated is compared and contrasted to the phylogeny of their originating species. If there are differences between the investigated gene and the phylogeny, it is reconciled by introducing Horizontal Gene Transfer. This approach allows us to identify ancient Horizontal Gene Transfer events in a genome but only works under strict restrictions on where to seek the Horizontal Gene Transfer in a genome.  

Show question

Question

What is the composition-based approach to detecting horizontal gene transfers?

Show answer

Answer

In a composition-based approach, the genome is compared and contrasted based on its properties.  

Show question

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