Transposable elements are short sequences of DNA with the special characteristic that they are mobile: they copy themselves to new locations on the genome.
While some have been co-opted to serve a function, transposable elements are largely parasitic, abusing the cellular machinery to aid their own propagation. They’re usually classified as selfish genetic elements.
There are generally three types of transposable element: DNA transposons, LINEs and SINEs, and LTR retroelements.
- DNA transposons are the simplest of the transposable elements. They encode for a protein, a transposase, that recognises the transposable element, and cuts and pastes it to a new area of the genome, usually near its previous position. Because this creates a gap in the DNA at the element’s previous position, DNA repair mechanisms are activated, which use the second strand as a template, resulting in the transposable element getting recreated at the gap. Hence there are now two transposable elements instead of one, and the genome becomes slightly longer. (This doesn’t happen every single time though, there are several mechanisms that can get activated.)
- LINEs (long interspersed nuclear elements) do not code for proteins to cut and paste them. Instead, they use RNA to get transposed. The element gets transcribed into RNA then translated, as a normal section of DNA. The translation product, however, binds to the mRNA that is doing the translating, and this complex goes back and cuts the LINE into the DNA. The precise mechanisms are, to my knowledge, not quite elucidated yet (a cell biologist will know better though).
- SINEs (short interspersed nuclear elements) are independent sections of DNA that evolved to take advantage of the LINE machinery to propagate themselves. Think of them as parasitic.
- LTR retroelements (long terminal repeat retroelements) are rather more complex, coding for two types of protein and three types of enzymes. Like in LINEs, the first step is to get transcribed to DNA, but from there, two things can happen:
- It gets translated into a protein, and then goes through a similar fate as a LINE.
- A protein it codes for gets activated after RNA transcription, causing the RNA to become encapsulated and reverse transcribed into the genome.