A very abundant type of transposons in our genome are the “retrotransposons” because they move their own DNA around the host’s DNA via an RNA intermediate (using “reverse transcription”). They are also know as “copy-and-paste” transposons, meaning they leave a copy of themselves behind and add a new one elsewhere. Retrotransposons are such a huge chunk of our DNA that they make on average about 40% of all mammalian genomic sequences. They account for most of the “repetitive DNA” found in eukaryotes, which used to be referred to as “junk” or “selfish” DNA when it was thought they had no relevant function and that they focused all their efforts on maintaining themselves in their hosts with no associated benefit for the latter. Another type of transposons that move around by a “cut and paste” mechanism are DNA transposons which comprise about 3% of the human genome.
These little DNA elements at times jumping around in our DNA reflect the real genomic scenario in which there is a dynamic interplay between a variety of factors and our DNA (epigenetics being a major mechanism of regulation of all these processes- see my homepage for details) that results in potential ways of generating changes that may be needed to face environmental or infectious threats, or alternatively, in damaging endogenous alterations as well. Cancer, in itself a stressor, has been shown to result in increased numbers of transposons in tumoral cells of ovarian, prostate, liver, and colon cancers, making them potentially involved with the initial process that led to the cancer (under investigation) and certainly making transposons number increase a “marker” for cancer cells.