Transposon

Transposons are sequences of DNA that can move around to different positions within the genome of a single cell, a process called transposition. In the process, they can cause mutations and change the amount of DNA in the genome. Transposons were also once called jumping genes, and are examples of mobile genetic elements. They were discovered by Barbara McClintock early in her career, for which she was awarded a Nobel prize in 1983.
There are a variety of mobile genetic elements, and they can be grouped based on their mechanism of transposition. Class I mobile genetic elements, or retrotransposons, copy themselves by first being transcribed to RNA, then reverse transcribed back to DNA by reverse transcriptase, and then being inserted at another position in the genome. Class II mobile genetic elements move directly from one position to another using a transposase to "cut and paste" them within the genome.
Transposons make up a large fraction of fieciesgenome sizes which is evident through the C-values of eukaryotic species. The sheer volume of seemingly useless material initially puzzled researchers, so that it was termed "Junk DNA" until further research revealed the critical role that it played in the development of an organism. They are very useful to researchers as a means to alter DNA inside a living organism.


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http://en.wikipedia.org/wiki/Transposon