This video, we're going to focus on plasmids. Plasmids can be defined as circular double-stranded DNA molecules with an origin of replication allowing them to replicate within a cell. Some plasmids are known as high copy number plasmids, and these are plasmids that will replicate very fast or very quickly inside a cell. Other plasmids are known as low copy number plasmids and so they replicate very, very slowly within the cell. These high copy number plasmids that replicate quickly, of course, are going to be found in high numbers or high copy numbers because they replicate so fast.
Whereas, the low copy number plasmids that replicate slowly are going to be found in relatively low numbers within the cell or low copy number. These plasmids, regardless of if they are high copy number or low copy number plasmids, can carry various genes. Sometimes some of those genes are going to be able to provide the cell with the ability to survive in a particular environment, such as an environment that contains antibiotics, for example. We call these plasmids resistance plasmids, otherwise known as r plasmids. Resistance plasmids or r plasmids are plasmids that encode genes that confer resistance to antibiotics, allowing the cell to survive in the presence of antibiotics.
These genes, that are found on the resistance plasmids, are sometimes referred to as r genes. In addition to the r genes, it's important to note that these plasmids can also have other genes. Most of the resistance plasmids are going to be conjugative plasmids, which means that these plasmids can be horizontally transferred between different species via conjugation. Other than the r genes, they can also contain other genes that are required for DNA transfer by conjugation. If we take a look at our image down below over here on the left-hand side, notice that the top half of this image is focusing on high copy number plasmids.
High copy number plasmids replicate very quickly within the cell, and so they're going to be found in relatively high numbers, high copy numbers within the cell. Whereas the low copy number plasmids on the bottom half over here, you’ll notice they replicate very slowly over time, and so they're going to be found in relatively low numbers within the cell. Here is a representation of a plasmid and some of the different regions of DNA that can be found within the plasmid. This is specifically an r plasmid here, a resistance plasmid. It’s a resistance plasmid because it contains specific genes that allow for antibiotic resistance, such as an ampr gene, which allows for ampicillin resistance, a gene that allows for ampicillin resistance, allowing the cell to survive in the presence of ampicillin.
At the top is a region, a gene, the tetr gene or tetracycline resistance, resistance to another antibiotic. Then in purple what we have is a canr gene providing resistance to an antibiotic called kanamycin. In addition to being an r plasmid, it's also a conjugated plasmid as well because it contains this tra region or tra region here, which is the region that is important for conjugation, allowing the r plasmid to be horizontally transferred and passed from cell to cell via conjugation. This also allows for the spreading of resistance genes between different species of bacteria. This concludes our brief lesson on plasmids and how they contribute to genome variability because they are mobile genetic elements that can be passed from one cell to another cell via conjugation.
We'll be able to talk about other mobile genetic elements as we move forward in our course. So, I'll see you all in our next video.