What is a DNA Plasmid? - Importance to Genetic Engineering

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  • 0:06 Plasmid Definition
  • 1:25 Multiple Cloning Site
  • 1:56 Origin of Replication
  • 3:21 Selectable Marker
  • 6:04 Antibiotic Selectable Marker
  • 7:26 Lesson Summary
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Lesson Transcript
Instructor: Greg Chin
DNA plasmids play an integral part in most genetic engineering experiments. In this lesson, you'll learn about key features of a plasmid, such as a multiple cloning site, an origin of replication, and a selectable marker.

What Is a Plasmid?

A plasmid is a small circle of DNA found in bacteria and is a vehicle for storing and studying genes.

Genetic engineering, by its nature, requires that DNA be stored until needed and moved at will from the test-tube environment to a cellular environment or vice versa. A DNA plasmid makes it easier to accomplish this goal. A plasmid is a small circle of DNA, typically found in bacteria, that is separate from the majority of bacterial DNA located in the nucleoid. That might sound like a mouthful, but basically a plasmid is a vehicle for storing and studying genes.

Scientists use plasmids in laboratory experiments as a vector, or vehicle, for DNA of interest, typically a gene. Plasmids are usually represented by a simple circle with the important features noted. To be an effective tool for scientists, a plasmid typically possesses three basic features: a multiple cloning site, an origin of replication, and a selectable marker. Let's see how each of these plasmid features plays a key role in genetically engineering bacteria to produce human insulin.

Multiple Cloning Site

If the main purpose of a plasmid is to serve as a vehicle for genes of interest, we need to be able to insert the human insulin gene into the plasmid. This is the purpose of a multiple cloning site. A multiple cloning site is the location in a plasmid where a sequence of DNA, typically a gene, can be inserted. We'll discuss in greater detail how a gene can be inserted into the multiple cloning site in another lesson.

Origin of Replication

Once a gene is inserted into a plasmid, you obviously don't want to lose it. Wouldn't it be great if the bacteria did all the work maintaining the plasmid for us? Actually, that's one of the main reasons scientists put a gene into a bacterial plasmid. As we learned previously, the origin of replication is the place where the process of DNA replication begins. Therefore, if our plasmid possesses an origin of replication, the bacteria will automatically make a new copy of the plasmid during the replication process.

DNA replication creates copies of genetically-engineered plasmids.
Replication process

Recall that DNA replication creates an exact copy of DNA in the cell. This means that the two daughter cells both receive a copy of the DNA found in the mother cell when the mother cell divides. If our genetically-engineered plasmid has an origin of replication, it is also replicated. This means that each daughter cell also receives a copy of our human insulin gene whenever the bacterial cell undergoes replication. In this way, our bacterial host has become a living storage vessel for the genetically-engineered plasmid. As long as this genetically-engineered bacteria is alive, it will maintain our insulin plasmid.

Selectable Marker

Okay. So, we have a means of getting the insulin gene into a DNA plasmid using the multiple cloning site. And, we will have a continual supply of our plasmid because the origin of replication will ensure that a new copy of the plasmid is created each time the host bacterial cell divides. That's great. With those plasmid characteristics alone, we can create genetically-engineered bacteria that will make insulin for us.

But, isn't bacteria all around us? How are we going to differentiate between the genetically-engineered bacteria and 'regular' bacteria from the environment? Right now, there's no way we'd be able to distinguish between the two. Since bacteria is everywhere and mass-producing insulin is going to entail use of optimal bacterial growth conditions, what's to prevent contaminating bacteria from interfering with industrial production or laboratory experiments?

The answer is a selectable marker. Consider the proverbial needle in a haystack. How do you propose finding the needle amongst all that hay? Obviously, we could sift through the haystack by hand 'til we find the needle, but that seems like a lot of work. Adding a magnet as a tool would be a more sophisticated version of this search method, but, at its heart, this is still a search as well. We're still actively doing something, albeit less work because of the magnet.

Wouldn't it be great if we could just sit on a porch drinking lemonade while the needle finds itself? How's this for an idea? Let's toss a match into the haystack and get that lemonade while we consider the consequences of our fiery plan. The fire is going to burn the hay to the ground, allowing us to just waltz up and pick up our needle once we've finished our lemonade. Note that we didn't need to do any work once we set the haystack ablaze.

This third characteristic of plasmids enables identification of genetically modified organisms.
Selectable marker

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