In this video, we're going to discuss the mechanism of natural transformation, which is one of the types of horizontal gene transfer. Cells that are naturally competent and able to transform or uptake DNA can regularly transform or uptake DNA from the environment when that DNA is available. The reason that DNA might be available in the environment is because maybe a nearby cell lysed or ruptured and released its DNA into the environment. Thus, natural transformation can really occur in a series of 4 steps that we have numbered down below: 1, 2, 3, and 4.
In these images, the blue structure represents a bacterial cell, and the circular structure inside represents the chromosome of the bacteria—the original DNA that belongs to that bacteria. You'll notice that this little segment of DNA represents the free naked DNA that is going to be transformed by the cell or uptake into the cell. This free naked DNA, in many cases, will contain genes. In this case, it contains an antibiotic resistance gene for antibiotic x, where x is just any antibiotic, just for the sake of an example. The antibiotic resistance gene is found in this external piece of DNA. This external piece of DNA is a double stranded DNA molecule, indicated as dsDNA, representing double stranded DNA.
The double stranded DNA molecule in the free naked DNA will simply bind to cell surface receptors on the surface of the bacterial cell. The ssDNA mentioned represents single stranded DNA. The single stranded DNA is going to enter the cell, and the complement strand, the other strand, is going to get degraded. You can see here that one of the strands is going to be broken up and degraded, whereas the other strand is actually going to enter the cell.
The single stranded DNA that enters the cell is then going to integrate into the host cell's chromosome, and it does this via homologous recombination. You can see this little single stranded piece of DNA that has entered and been transformed is going to integrate into the chromosome via homologous recombination. You can see the integration here. In the 4th and final step, the cell is going to rebuild the complement strand, the other strand that was degraded. It rebuilds that, so now we have a double stranded region back in place. This antibiotic resistance gene has now been incorporated into the chromosome, and so now the cell has resistance to antibiotic x because it obtained this antibiotic resistance gene through horizontal gene transfer, through natural transformation.
This concludes our brief lesson on the mechanism of natural transformation. We'll be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.