Transposable Elements in Prokaryotes - Online Tutor, Practice Problems & Exam Prep

Prokaryotic Transposable Elements

So prokaryotic cells have two types of transposable elements. The very first type is called an insertion sequence. And what is an insertion sequence? It's going to be a short bacterial DNA sequence that jumps around the genome because it's a transposable element, and we know transposable elements jump. So, the important part to know about all transposable elements, prokaryotic ones, eukaryotic ones, doesn't matter, is that there's an enzyme that allows them to jump, and this enzyme is called transposase. This is the protein that's required for the movement of the insertion sequence, but it's also important for the movement of all different kinds of transposable elements. And so, we're going to see that different transposable elements control transposase a little bit differently. We're going to talk about the insertion sequence. An insertion sequence has two main structural features. The first is that they contain inverted repeat sequences at each end. These are actually a very common feature in transposable elements, and these repeats are required for the gene to be able to jump. So super important one. And then the second is the transposase. In insertion elements, how they control that transposase activity is they actually encode it between the inverted repeats. So what this looks like is, here's an inverted repeat, and you can see why it’s called that way because these are inverted repeats. Right? This is the same sequence, but backwards over here. And if this was an insertion sequence, the transposase gene would sit here. And when this is transcribed, the transposase would be transcribed with it. We process, translate, produce into a protein. The second type is called just a transposon, and it's probably what you're more familiar with hearing. These are longer DNA sequences compared to the insertion elements that also jump around the genome. And there are two types of these. So the first type of transposon is called a composite transposon. Composite transposons are flanked by insertion sequences. And as I said before, insertion sequences were flanked by inverted repeats, but composite transposons actually have two insertion sequences on either end. And the insertion sequences, which we talked about before, encode transposase. So the composite transposons don't actually encode for transposase, themselves. They have the insertion sequences on either side that encode for transposase, so they don't need to. This is how a composite transposon handles transposase differently. And then there are different genes that sit in between those insertion sequences. The second type is the simple transposon. These are flanked by inverted repeats, and that's around genes that include transposase. And these look much more similar to insertion sequences, except they tend to be longer. But, this looks very different. So, generally, the important part about transposon, we always talk about drug resistance in bacteria. Right? Oh, that's a big thing in the news right now, these superbugs. Well, generally, drug resistance is actually conferred and transferred between different bacteria based on transposons found on these bacterial r plasmids. And that's just a fun fact. You don't necessarily need to know it, but just fun fact, transposons are really what's responsible for these superbugs that we are developing now. So here we have a composite transposon. These are the insertion sequences. Remember, the insertion sequences encode for transposase. So, they're already present here, so we don't need transposase in the genes. And the simple transposon instead, has inverted repeats. The inverted repeats don't have transposase themselves, so the transposon has to actually encode for transposase. So, you can see that here in red. Now, transposon, so we're still in this second type of transposable element. So transposon, transpose, that's the verb of actually jumping, in two main ways. You can see that we like the two pattern here. So, and within transposon, there are two ways to jump, and the first is called replicative transposition, and that is when the transposon is actually copied, so it's replicated. And then the new copy travels to the new location while the old copy remains in the same location. So you can think of replicative transposition as copy and paste. So, there's now, after this copy and paste has happened, there are now two of the exact same type of transposons just located in different regions of the genome. Then, the second type is conservative transposition, and this is where the transposon is cut out and moved to a new location. So, this is the cut-and-paste method, where after the jump you still have the same number of transposons, it's just moved to a new location. So the two types are the copy and paste and the cut and paste. And those are, you know, make sure you understand that difference. So if we have the copy and paste method, what happens is this gets replicated, and this old one stays in the same spot, and the new one moves somewhere else in the genome. Whereas in conservative transposition, if you start out like this, what happens is it's just cut out and it moves to a new location when you still only have one copy. And this, you end up with two copies. So the copy and paste, and this is the cut and paste method. And, obviously, those have different impacts on the genome where this one keeps replicating itself, getting more and more copies, whereas this one always stays the same number of copies. So, with that, let's now move on.

Transposase is a protein that is responsible for what?

• A. Excising a transposable element from a donor site
• B. Insert a transposable element into a target site
• C. Degrading a transposable element that is harming the host genome
• D. Amplifying a transposable element

Which of the following transposons do not encode for the transposase enzyme?

• A. Insertion sequence elements
• B. Composite transposons
• C. Simple transposons

Which of the following sequences is an example of an inverted repeat sequence that would surround one strand of an insertion sequence element?

• A. 5' AATCG CGATT
• B. 5' AATCG AATCG
• C. 5' AATCG TTAGC
• D. 5' AATCG GCTAA