Horizontal Gene Transfer: Videos & Practice Problems

Horizontal gene transfer (HGT) enables bacteria to acquire new traits through three mechanisms: transformation, transduction, and conjugation. Transformation involves the uptake of free DNA, while transduction is mediated by bacteriophages transferring DNA between cells. Conjugation requires direct cell-to-cell contact for DNA transfer. Following HGT, the transferred DNA can integrate into the chromosome, remain as a self-replicating plasmid, or be degraded. Homologous recombination facilitates the integration of similar DNA sequences, enhancing genetic diversity and adaptability in bacterial populations.

Horizontal Gene Transfer

In this video, we're going to begin our lesson on horizontal gene transfer. Recall from our previous lesson videos that horizontal gene transfer occurs between two organisms that are not direct descendants of one another. These two organisms are going to be transferring genes between each other. Horizontal gene transfer allows cells to quickly acquire new traits and also drives genetic diversity among organisms. Now, there are three known mechanisms of horizontal gene transfer in bacteria. Notice that we have those three mechanisms numbered down below: 1, 2, and 3. The first mechanism is transformation, the second mechanism is transduction, and the third mechanism of horizontal gene transfer is conjugation. You'll notice that these numbers also correspond with the numbers in our image below.

For the first mechanism of horizontal gene transfer, we have transformation. Transformation is horizontal gene transfer via the uptake of free or naked DNA in the environment by a cell. If we take a look at our image down below at number 1 on the left-hand side, notice it's showing you transformation, where a cell brings in free or naked DNA from the environment. When the free or naked DNA is brought into the cell and incorporated into the cell, we refer to this as transformation. Later in our course, we'll be able to talk more about transformation.

Transduction is going to be a form of horizontal gene transfer mediated specifically by a bacteriophage virus. A bacteriophage is a specific type of virus that infects bacteria. If we take a look at number 2 down below, notice that this is showing us transduction. While they sound similar, transduction is different from transformation. Here, a bacteriophage or phage is responsible for transferring DNA into the cell. You can see donor cell DNA being transferred by the bacteriophage, and then the recipient cell can receive that DNA. We'll get to talk more about transduction later in our course. This is just the introduction to it.

The third and final mechanism of horizontal gene transfer is conjugation, referring to the direct horizontal DNA transfer between cells during direct cell-to-cell contact. Notice in the final image over here focusing on conjugation. You will see two neighboring cells forming direct cell-to-cell contact where DNA and genes can be transferred from one organism over to a neighboring organism. Notice that the green molecule, originally not in the cell on the right, is now in both cells at the end. This is an example of conjugation, the transfer of genes via direct cell-to-cell contact. We'll be able to talk more about conjugation later in our course. This is just the introduction.

This concludes our brief introduction to horizontal gene transfer, and I'll see you all in our next video.

Which of the following is not a type of horizontal gene transfer?

• A. Transduction.
• B. Transformation.
• C. Transamination.
• D. Conjugation.

Answer: (C)

Fates of Horizontally Transferred DNA

In this video, we're going to talk about the fates of horizontally transferred DNA. Or in other words, we're going to talk about what happens to the DNA that is horizontally transferred into an organism. Following horizontal gene transfer, there are three possible fates of the DNA. We're showing you these three possible fates in the image below. At the top, you'll notice a bacterial cell. The blue represents the chromosomal DNA, which is the original DNA belonging to the bacteria. On the right, we're showing the horizontally transferred DNA, which is obtained by the cell via horizontal gene transfer.

This horizontally transferred DNA can have one of three fates in the cell. The first fate is that the horizontally transferred DNA is integrated into the chromosome and replicates alongside the chromosome. This horizontally transferred DNA can integrate and become a part of the chromosomal DNA, which is shown below as the integration of that horizontally transferred DNA. The second fate is that the horizontally transferred DNA remains self-replicating, without actually integrating into the chromosome. An example of this would be a plasmid, which is a small, circular, extrachromosomal segment of DNA. It's separate from the chromosome and is self-replicating. It can replicate on its own without having to integrate. That is another option for the horizontally transferred DNA.

The third possible fate for the horizontally transferred DNA is that it is actually degraded. The degraded, horizontally transferred DNA will end up having no effect. If the cell degrades it into pieces like this, eventually, this will have no effect on the cell. Therefore, only options 1 and 2, which are the integration of the DNA or self-replicating DNA, will stabilize the transferred gene within the population and allow this bacteria to pass those genes to future generations as it replicates. These are some important fates to keep in mind regarding horizontally transferred DNA as we move forward.

That concludes this video, and I'll see you all in our next one.

All of the following are possible outcomes of horizontally transferred DNA, except which of these answers?

• A. The transferred DNA is turned into a plasmid and replicates with the bacterial cell.
• B. The transferred DNA triggers the degradation of the bacterial chromosome.
• C. The transferred DNA is integrated into the bacterial chromosome and is replicated with the chromosome.
• D. The transferred DNA is degraded by the bacterial cell and has no effect on the cell.

Answer: (B)

When DNA is transferred between bacterial cells, there are three possible fates of the transferred DNA. Which fate ensures that the transferred DNA will be retained inside of the bacterial cell the longest?

• A. The transferred DNA being integrated into the bacterial chromosome.
• B. The transferred DNA forming a plasmid within the bacterial cell.
• C. The transferred DNA being degraded by the bacterial cell.

Answer: (A)

Integration of DNA via Homologous Recombination

In this video, we're going to briefly discuss the integration of DNA via homologous recombination. Homologous recombination is really just referring to the process of genetic exchange between two similar strands of DNA. It allows molecules to integrate into other segments of DNA. Homologous recombination can only occur if the donor DNA actually has a similar nucleotide sequence to the recipient cell's chromosome.

If we take a look at our image down below, we can get a better understanding of homologous recombination. What you'll notice on the left is the chromosomal DNA of, say, the bacteria. The chromosomal DNA is the original DNA that's found in the bacteria. What we're showing you down below is the donor DNA that could have potentially been obtained through horizontal gene transfer. The donor DNA has some regions highlighted in orange that have similar sequences to the chromosomal DNA. These yellow highlighted regions represent areas of similarity. When these regions have similarity, it creates the possibility for homologous recombination.

What can happen is the donor DNA, which I'll highlight here like this, can be incorporated and replace the chromosomal DNA at this region. The chromosomal DNA can be removed, whereas the donor DNA can be integrated. This is what we are seeing here. The donor DNA can be integrated into the chromosomal DNA and it replaces a region of the chromosomal DNA. This is what we call homologous recombination, and it can allow donor DNA to be incorporated and integrated into the chromosomal DNA.

This here concludes our brief lesson on the integration of DNA via homologous recombination. We'll be able to get some practice applying this as we move forward. So I'll see you all in our next video.

Homologous recombination:

• A. Can occur in prokaryotes and eukaryotes.
• B. Occurs when two similar strands of DNA exchange genetic material.
• C. Only occurs in prokaryotes when the donor DNA has the same sequence as the recipient's DNA.
• D. All of the above are true about homologous recombination.

Answer: (D)

Here’s what students ask on this topic:

What is horizontal gene transfer and why is it important?

Horizontal gene transfer (HGT) is the process by which an organism transfers genetic material to another organism that is not its offspring. This is crucial for bacteria as it allows them to rapidly acquire new traits, such as antibiotic resistance, and adapt to changing environments. HGT contributes to genetic diversity and evolution, enabling bacteria to survive in various conditions. The three main mechanisms of HGT are transformation (uptake of free DNA), transduction (DNA transfer via bacteriophages), and conjugation (direct cell-to-cell DNA transfer).

What are the three mechanisms of horizontal gene transfer in bacteria?

The three mechanisms of horizontal gene transfer in bacteria are:

1. Transformation: Uptake of free or naked DNA from the environment by a bacterial cell.

2. Transduction: DNA transfer mediated by bacteriophages, which are viruses that infect bacteria.

3. Conjugation: Direct transfer of DNA between two bacterial cells through direct cell-to-cell contact.

These mechanisms enable bacteria to acquire new genetic traits and enhance their adaptability and survival.

What happens to DNA after horizontal gene transfer?

After horizontal gene transfer, the transferred DNA can have one of three fates:

1. Integration: The DNA integrates into the bacterial chromosome and replicates with it.

2. Self-replication: The DNA remains as a self-replicating plasmid, independent of the chromosomal DNA.

3. Degradation: The DNA is degraded and has no effect on the cell.

Only the first two fates stabilize the transferred gene within the population, allowing it to be passed on to future generations.

How does homologous recombination facilitate the integration of DNA?

Homologous recombination is the process of genetic exchange between two similar DNA strands. It allows the integration of donor DNA into the recipient's chromosome if the sequences are similar. During this process, the donor DNA replaces a corresponding segment of the chromosomal DNA. This integration enhances genetic diversity and adaptability in bacterial populations by incorporating new genetic traits.

What is the role of bacteriophages in horizontal gene transfer?

Bacteriophages, or phages, are viruses that infect bacteria and play a crucial role in horizontal gene transfer through a process called transduction. During transduction, a bacteriophage transfers DNA from one bacterial cell (the donor) to another (the recipient). This transfer can introduce new genetic material into the recipient cell, contributing to genetic diversity and the acquisition of new traits.