Transposable elements are segments of DNA that are capable of moving around the genome. They were first discovered by Barbara McClintock in 1948 through her work with corn. The color of the corn kernels at right is due to a transposon. This causes some kernels to be purple, some yellow, and some a mosaic of both. About 45% of our genome is composed of transposons. What exactly are they, and what do they do?
Class I Transposons (Retrotransposons)
There are two classes of transposable elements. The first is known as Class I, Retrotransposons, or Copy-and-Paste Transposons.
- LTR (Long Terminal Repeats) - Retrotransposons
- Non LTR-Retrotransposons: LINEs and SINEs
- DNA is transcribed to RNA.
- RNA is converted back to DNA by reverse transcription.*
- DNA is inserted back into the genome.
*Retrotransposons act very similarly to retroviruses, like HIV.
Outcome: The transposon is now in two locations.
Class II Transposons
The second class of transposable elements is known as Class II, DNA Transposons, and Cut-and-Paste Transposons.
- Transposase cuts the transposable element out of the DNA.
- DNA ligase inserts the DNA into a different location.
Outcome: The transposon has moved.
Impact on the Genome
The number of transposable elements in the genome is constantly increasing. As one would imagine, this can cause a number of problems. Some have even gone so far as to call transposons parasitic DNA. They have no purpose, but they can affect the genome.
What impacts do they have?
- Transposons may insert into genes. This can cause the gene to lose function and can damage the organism.
- They increase the time it takes for DNA replication because the amount of DNA has increased.
- In rare instances, the transposons themselves are functional.
- They increase genetic diversity.