What are Transposable Elements?

Transposons are mobile DNA elements found in the genome of all organisms and therefore have become known as "jumping genes".  Barbara McClintock first discovered transposable elements in maize in 1944 at Cold Spring Harbor Laboratories.  Over the years many transposable elements have been discovered, yet despite their abundance and tendency to randomize genome order, the gene order amongst different species appears to be fairly well conserved.  The effects of transposable element insertion and excision depends heavily on the location within in the coding sequence.  For instance, insertion or excision of an exon commonly causes either an altered gene product or a frame-shift mutation.  

Transposons are divided into Class I transposons and Class II transposons.  Class I transposons use a RNA intermediate which is encodes a reverse transcriptase/integrase to help them transpose.  Class II transposons are known for their "cut and paste" method in which they use transposase/integrase to enable their movement.  Further classification can be made of the Class I transposons into long terminal repeat (LTR) retrotransposons, long interspersed nuclear elements (LINEs), and short interspersed nuclear elements (SINEs).  LTR retrotransposons and miniature inverted-repeat transposable elements (MITEs) are important components of plant genes and genomes.  An inverted repeat means that the base pairs have opposite orientation.  

For example:  AAAATTTAGCGCG and CGCGCTAAATTTT are inverted repeats.

Short DNA transposons as well as short interspersed nuclear elements (SINEs) tend to be more abundant in eukaryotes than the longer ones. One explanation is that shorter insertions may have less impact on host phenotype and therefore are more easily tolerated. Indeed, short Transposable Elements, including MITEs and SINEs, appear to be associated with gene rich rather than with gene poor regions.

We have provided sequence data for MITEs and LINEs that you can use in the "Blast Off" section of this site.