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A transposable element, or transposon, is a mobile piece of genetic material. These sequences of deoxyribonucleic acid (DNA) are either replicated multiple times within a genome or are moved around in the original form. Due to their ability to create new genes, transposable elements are considered mutagens and are an import device in evolution. Transposable elements are being studied to help gain a better understanding of genetic change and the causes of disease.
American scientist Barbara McClintock first discovered the transposable element around the end of the Second World War. She studied the methods of reproduction of maize, or corn, with a focus on how chromosomes change. She also produced the first genetic map for corn. Her research was initially met with skepticism. It wasn’t until 1983 that she was awarded a Nobel Prize for her work.
The first possible method of mobility for a transposable element is similar to the “copy and paste” function on a computer. These types of transposable elements are categorized as Class I and are sometimes called retrotransposons. A ribonucleic acid (RNA) intermediary is used in this replication mode. Retrotransposons are particularly abundant in plants and other eukaryotes, or organisms with complex cells. Nearly half of the human genome consists of this form of transposable element.
DNA transposons are the second type of transposable element, categorized as Class II. Rather than using an RNA intermediate, Class II elements generally use enzymes in a process similar to “cut and paste.” Enzymes are types of molecules that help speed up chemical reactions in an organism. DNA transposons are less common in the human genome than retrotransposons, but still play an important role in evolution.
Transposable elements are known to cause diseases. Hemophilia A and B, predisposition to cancer, and a type of muscular dystrophy can all be caused by this kind of change to genetic material. If a transposable element is inserted into a functional gene, it can disable the whole gene. If a gap in a gene is left by an exiting DNA transposon, the gene is usually not correctly repaired.
Cells typically have defensive mechanisms against excessive genetic change. Bacteria can regularly delete large portions of their genome as a guard against the propagation of viruses and transposable elements. Eukaryotes may use RNA molecules in their cells to interfere with transposable element activity. These measures are evolutionary adaptations to help keep new mutations in check.
The evolution of transposable elements is not well understood. Some believe the phenomenon arose early in the evolutionary history of life and was passed down to later species. Others contend that the elements originated multiple times independently of each other. Another possibility is that transposable elements could have evolved more recently and spread to different forms of life through a process called horizontal gene transfer. In any case, transposable elements are found in all major branches of life today.
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