Transposable elements fall under the category of dispersed sequence repeats. Transposable elements are the sections of DNA that move or transpose within the genome. They are also sometimes referred to as molecular parasites, as they exist only to maintain and propagate themselves and have no specific function in the biology of their host. However, some reports have shown that they are involved in the evolution of genomes (PNAS, 2000). Reports have also shown that they are involved in some important cellular functions. These elements are of two kinds: Transposons and retrotransposons.
Transposons are mobile DNA elements that transpose directly as DNA molecule. The Ac/Ds system (to be discussed in the next post) and the P elements of Drosophila are the two prominent examples of transposons. The transposase enzyme is required for their movement.
Retrotransposons move via an RNA intermediate using the reverse transcriptase enzyme. The yeast Ty elements and the copia elements of Drosophila are the examples of retrotrasnposons.
Mobile elements are eliminated from the genome at a very slow rate. They are removed either by deletion of DNA segments carrying them or by accumulation of mutations. Mobile elements also show evidence of environmental regulation as seen for Tam elements in snapdragon (Antirrhinum majus). The frequency of Tam element transposition increases by 1000-fold when snapdragon plants are shifted from 25°C temperature to 15°C.