So, eukaryotic cells have 2 types of transposable elements. The first type is retrotransposons, otherwise called class 1 elements. Either way, they're the exact same thing. Retrotransposons are referred to this way because they use an RNA intermediate to jump. What does this mean? Initially, you start off with DNA, that DNA is then transcribed and now it's RNA, and then that RNA is turned back into DNA and then jumps. The RNA intermediate is needed for that jumping. Often, retrotransposons have their evolutionary history in RNA viruses, also called retroviruses. RNA viruses have genomic material that is just RNA. It's single-stranded, which is what "ss" stands for, single-stranded RNA, and that's their genetic material. But that RNA also encodes for something called reverse transcriptase, which we may have mentioned before. Reverse transcriptase is a protein responsible for transcribing RNA into DNA. So it is backward, which is why it is called reverse, because usually, transcription is DNA to RNA; reverse transcription is RNA to DNA. So, typically how these evolved is, you had a virus that was single-stranded RNA. It infected the cell. That RNA was reverse transcribed to DNA, and then it was integrated into the genome. Because it was DNA, the genome was like, 'Hey, there's some free DNA. Let me take that up.' That DNA is called something special when the virus puts it in there. It is called a provirus, which is the DNA from the virus that integrates into the genome. Essentially, how this works is you start off with RNA, go to DNA, and then it is inserted into the genome. An example of a retrotransposon is called a long terminal repeat retrotransposons, and this means exactly what it sounds like. It is a retrotransposon, meaning that it jumps via an RNA intermediate, but it has these long repeats on the end. And an LTR retrotransposon has these repeats on the end. Long terminal repeat transposons use the copy and paste method we talked about before to transpose.
Then the second class of transposable elements are DNA transposons or class 2 elements, and these are the ones that we are more familiar with; they are very similar to the way that the prokaryotic one jumps and they use DNA to jump. Here we have an example. Here is an LTR retrotransposon. These long terminal repeats on either end are transcribed into RNA. RNA is reverse transcribed into DNA. That DNA then integrates elsewhere in the genome, and that is how the retrotransposons actually end up jumping. So, with that, let's now turn the page.