What are Transposable Elements?

Transposable Elements (sometimes also called transposons or mobile DNA; referred to from now on as TEs) are small sections of DNA which have the ability to move around the genome.

TEs were first discovered by Barbara McClintock around 1950. Her discovery of transposable elements was not immediately understood or accepted by her contemporaries. As we have learned more about the structure of the genome and about DNA in general, we have come to understand more about these interesting elements, although they can still be very confusing.

The first thing to know about TEs is that they mostly don't have a function in the host organisms (as far as we are aware). Some individual elements have been co-opted as part of a gene, or a part of the control mechanisms for switching genes on or off and it has also been suggested that TEs help to regulate the size of the genome, but in general, they are 'selfish DNA', they exist just to replicate themselves.

The second thing to know about TEs in that they take up a huge proportion of our genome. Far more than the protein coding genes do.

Image taken from http://bio1510.biology.gatech.edu/module-4-genes-and-genomes/4-8-genomes/

In the image above those segments labelled Lines, Sines, LTR retrotransposons and DNA transposons are all TEs.

How do TEs move?

There are two main types of TEs: DNA TEs and RNA TEs.

DNA TEs are often referred to as cut-and-paste elements. This is because the DNA is removed from its original position and inserted into a new postition. Often this results in elements moving around without increasing in number, however if the cut-and-paste event (called transposition) occurs during the right part of the cell cycle, when the DNA is being copied, then it can result in two elements, one in the original position and one in the new position.

RNA TEs are referred to as copy-and-paste. These elements created a template of themselves using RNA, much like protein coding genes do.  This then leaves the original element where it is and a new piece of DNA is created using the template, the new DNA is inserted in a different position in the genome. This means that this type of TE always increases in number when a transposition event occurs.