Transposable Element

A transposable element, or transposon, is a segment of DNA that can move from one location in the genome to another. Often called "jumping genes," they were discovered by Barbara McClintock in maize in the 1940s, and she received the 1983 Nobel Prize in Physiology or Medicine for the work.

DNA transposons use an enzyme called a transposase to cut themselves out and paste in at a new site. Retrotransposons copy themselves by making an RNA copy that is converted back into DNA by reverse transcriptase, a "copy and paste" mechanism that increases their number. Some, like retroviruses, are related to viruses, and may have common origins.

Nearly half of the human genome is made of transposable elements or their remains, mostly ancient copies that no longer move. The Alu element alone makes up about 10 percent and LINE-1 about 17 percent, as revealed by the Human Genome Project. In maize and wheat, the fraction is much higher, over 80 percent in maize.

Jumping into a gene can cause a mutation, and new insertions are a cause of some diseases such as hemophilia. Over evolutionary time, they have been a source of new regulatory sequences and genes, and contribute to gene duplication. Cells keep them in check with epigenetic silencing, including DNA methylation, and with small RNAs. Their spread is a classic example of genetic parasites in evolution.

• Genome — where they live
• Reverse Transcriptase — the enzyme retrotransposons use
• Epigenetics — how they are silenced